Overview

Huntingtin (HTT) is a large (~350 kDa) multi-domain protein encoded by the HTT gene on chromosome 4p16.3. While named for its role in Huntington’s disease (HD), huntingtin is an essential protein with fundamental functions in embryonic development, neuronal physiology, and cellular homeostasis. The protein contains a polymorphic polyglutamine (polyQ) tract in its N-terminus, and expansion of this tract beyond 35-39 CAG repeats causes Huntington’s disease, one of the most common neurodegenerative disorders.

Pathway / Mechanism Diagram

flowchart TD
    A["Gene<br/>Expression"] --> B["Huntingtin  (HTT)<br/>Protein"]
    B --> C["Protein<br/>Folding and Structure"]
    C --> D["Biological<br/>Activity"]
    D --> E["Cellular<br/>Function"]
    F["Regulation/<br/>Modification"] --> D
    E --> G["Normal<br/>Physiology"]
    B -->|"mutation"| H["Pathological<br/>State"]
    H --> I["Disease<br/>Phenotype"]

Introduction

Huntingtin is a fascinating protein that exemplifies both the normal functions of a large scaffold protein and the pathogenic consequences of specific genetic mutations. This ~3,144 amino acid protein is expressed ubiquitously but is particularly abundant in the brain, where it participates in numerous cellular processes essential for neuronal survival and function. This comprehensive page covers the structure, normal functions, disease mechanisms, and therapeutic strategies related to huntingtin. ^{1The Huntington's Disease Collaborative Research Project. A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes1993 · Cell · PMID 8458085Open reference}

^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}

Molecular Structure

Domain Organization

Huntingtin is organized into multiple functional domains:

• Polyglutamine (polyQ) tract (N-terminus, residues 1-60): The first exon contains a polymorphic CAG repeat encoding glutamine. Normal alleles have 10-35 repeats. Pathogenic expansions (>36 repeats) cause Huntington’s disease, with earlier onset associated with longer repeats.

• Polyproline (polyP) tract: Adjacent to the polyQ tract, this region mediates protein-protein interactions through SH3 domain binding.

• HEAT repeat domains (residues 600-2800): Huntingtin contains 36 alpha-helical HEAT (Huntingtin, Elongation factor 3, A subunit of PP2A, Tor) repeats that form elongated superhelical structures. These repeats mediate interactions with numerous partner proteins.

• Nuclear localization signals (NLS): Multiple NLS sequences facilitate huntingtin’s shuttling between cytoplasm and nucleus.

• Nuclear export signals (NES): Hydrophobic sequences enabling export from the nucleus.

• Caspase cleavage sites: Multiple Asp-Glu-Val-Asp (DEVD) sequences are recognized by caspases (particularly caspase-3 and caspase-6), generating toxic fragments in HD.

Post-Translational Modifications

Huntingtin is extensively modified:

• Phosphorylation: Over 100 phosphorylation sites identified. Key sites include S421 (neuroprotective), S116, S265, and T3.

• Acetylation: At Lys444, acetylation enhances mutant HTT clearance via autophagy.

• Sumoylation: Modification that can influence aggregation and transcriptional regulation.

• Palmitoylation: Affects membrane association and vesicular trafficking.

Normal Physiological Functions

Developmental Functions

• Embryonic development: HTT knockout is embryonic lethal in mice, indicating essential role in development

• Cell survival: Anti-apoptotic functions through multiple mechanisms

• Cellular transport: Facilitates vesicular transport along microtubules

Neuronal Functions

Synaptic Transmission

• Synaptic vesicle dynamics: Regulates synaptic vesicle pooling, release, and recycling

• Receptor trafficking: Controls AMPA, NMDA, and GABA receptor trafficking to the plasma membrane

• Presynaptic functions: Regulates synapsin phosphorylation and vesicle mobilization

Transcriptional Regulation

Huntingtin interacts with numerous transcription factors:

• REST/NRSF: Sequesters REST in the cytoplasm; loss of HTT leads to REST nuclear translocation and repression of neuronal genes

• p53: Modulates p53 transcriptional activity and apoptosis

• NCoR/SMRT: Co-repressor complexes involved in neuronal gene expression

• CBP/p300: Histone acetyltransferases affected in HD

Axonal Transport

• Kinesin/dynein interactions: Serves as a scaffold for motor proteins

• Vesicle trafficking: Transport of synaptic vesicles, neurotrophic factors (BDNF), and organelles

• Mitochondrial trafficking: Coordination of mitochondrial distribution in neurons

Autophagy Regulation

• Selective autophagy: Interacts with autophagy receptors (p62, OPTN)

• Lysosomal function: Regulates autophagosome-lysosome fusion

• Cargo recognition: Facilitates clearance of damaged organelles and protein aggregates

Neurotrophic Support

• BDNF trafficking: Critical for axonal transport of brain-derived neurotrophic factor

• TrkB signaling: Modulates neurotrophin receptor activation

Role in Huntington’s Disease

Genetics

• Inheritance: Autosomal dominant, full penetrance

• Repeat instability: Maternal and paternal expansion can occur, particularly paternal transmission

• Anticipation: Earlier onset in successive generations

• Modifier genes: Genetic modifiers influence age of onset (e.g., DNA repair genes)

Pathogenic Mechanisms

Gain-of-Function Toxicity

1. Transcriptional dysregulation: Mutant HTT (mHTT) sequesters transcription factors (CBP, p53, REST) in aggregates, disrupting gene expression

2. Mitochondrial dysfunction:

   • Impaired mitochondrial dynamics (fusion/fission)

   • Reduced respiratory chain activity

   • Increased ROS production

   • Disrupted calcium handling

3. Excitotoxicity:

   • Enhanced NMDA receptor activity

   • Impaired glutamate transport

   • Calcium dysregulation

4. Protein aggregation:

   • mHTT forms insoluble aggregates in nucleus and cytoplasm

   • Disrupts cellular transport, transcription, and organelle function

5. Axonal transport defects:

   • Impaired BDNF delivery

   • Synaptic vesicle depletion

   • Reduced neurotransmitter release

6. Autophagy impairment:

   • Defective autophagosome formation

   • Reduced clearance of damaged organelles

   • Accumulation of protein aggregates

Loss-of-Function

• Reduced normal HTT activity compounds toxicity

• Impaired BDNF transport

• Decreased neuroprotective signaling

Neuropathology

• Striatal degeneration: Medium spiny neurons (MSNs) in caudate and putamen are most vulnerable

• Cortical involvement: Layer 5 pyramidal neurons show atrophy

• White matter changes: Diffusion abnormalities in HD mutation carriers

• Other regions: Thalamus, hippocampus, cerebellum affected in later stages

Therapeutic Strategies

HTT-Lowering Approaches

Antisense Oligonucleotides (ASOs)

• Mechanism: Single-stranded DNA analogs that bind mRNA and promote RNase H degradation

• Clinical trials: Several ASOs have entered clinical trials (e.g., Tominersen, others)

• Challenges: Delivery to brain, allele specificity, timing of intervention

RNA Interference (RNAi)

• shRNA/siRNA delivery: Viral vectors (AAV) expressing hairpin RNAs

• Allele-specific targeting: Exploiting SNP differences between alleles

CRISPR-Based Approaches

• Gene editing: Correcting the expansion or reducing HTT expression

• Epigenetic modulation: Modifying DNA methylation or histone marks

Aggregation Inhibitors

• Small molecules: Designed to prevent or disrupt HTT aggregation

• Peptide-based inhibitors: Designed aggregation-blocking peptides

Modulation of Post-Translational Modifications

• Phosphorylation modulators: S421 phosphorylation enhancers

• Acetylation modifiers: K444 acetylation to promote clearance

• Proteostasis enhancers: Activating autophagy pathways

Neuroprotective Strategies

• BDNF augmentation: Enhancing neurotrophic support

• Metabolic support: CoQ10, NAD+ precursors, creatine

• Excitotoxicity blockers: Memantine, amantadine

• Mitochondrial protectants: Antioxidants, mitochondrial dynamics modulators

Animal Models

Genetic Models

• knock-in mice: CAG repeat expansions knocked into endogenous Htt locus

• Transgenic models: Expressing full-length or fragment mHTT (R6/1, R6/2)

• Yeast artificial chromosome (YAC) mice: Large genomic fragments with mutant HTT

Toxic Models

• Fragment models: N-terminal fragments with expanded polyQ

• Inducible models: Temporal control of mHTT expression

Cross-Pathology Connections

Parkinson’s Disease

• HTT mutations are not typical PD risk factors

• Common pathways: mitochondrial dysfunction, protein aggregation, autophagy impairment

• Potential for shared therapeutic approaches

Alzheimer’s Disease

• HTT can influence APP processing and Aβ generation

• Shared transcriptional dysregulation mechanisms

• Common therapeutic targets in protein homeostasis

Amyotrophic Lateral Sclerosis (ALS)

• Overlapping RNA processing abnormalities

• Shared defects in protein homeostasis

• Similar aggregate pathology

Biomarkers and Outcome Measures

• Motor markers: Unified Huntington’s Disease Rating Scale (UHDRS)

• Cognitive markers: Neuropsychological testing batteries

• Neuroimaging: Striatal volume, white matter integrity

• Biochemical markers: Neurofilament light chain (NfL), mutant HTT in CSF

Research Directions

• Genetic modifiers: Understanding what modifies age of onset

• Biomarker development: Identifying reliable progression markers

• Combination therapies: Multi-target approaches

• Premanifest intervention: Treating before symptoms emerge

Background

The study of Huntingtin Protein (Htt) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

See Also

• Alzheimer’s Disease

• Amyloid Hypothesis

• Tau Pathology

• Parkinson’s Disease

• Alpha-Synuclein

External Links

• PubMed - Biomedical literature

• Alzheimer’s Disease Neuroimaging Initiative - Research data

• Allen Brain Atlas - Brain gene expression data

Additional Content (merged from /entities/huntingtin-protein)

Huntingtin Protein (HTT)

Introduction

huntingtin (HTT) is a large, multifunctional protein of approximately 350 kDa encoded by the HTT gene on chromosome 4p16.3. The landmark discovery in 1993 by the Huntington’s Disease Collaborative Research Group of a CAG [trinucleotide-repeat-expansion in HTT as the cause of huntington-pathway (HD) transformed our understanding from a clinical syndrome to a molecularly defined genetic disorder ([The HD Collaborative Research Group, 1993)^{3Huntington's disease: seeing the pathogenic processes through a genetic lens2006 · Trends in Biochemical Sciences · PMID 16854264Open reference}90585-E)). HTT is one of the largest [proteins in the human proteome at 3,144 amino acids and serves as a critical scaffold for intracellular signaling, vesicular transport, and transcriptional regulation across many neuronal and non-neuronal tissues [^18]). ^{3Huntington's disease: seeing the pathogenic processes through a genetic lens2006 · Trends in Biochemical Sciences · PMID 16854264Open reference}

The mutant form of huntingtin causes Huntington’s Disease, a hereditary neurodegenerative disorder.

Overview

Huntingtin is ubiquitously expressed but is most abundant in the brain, particularly in neurons of the cortex, striatum, hippocampus, and cerebellum ^{1The Huntington's Disease Collaborative Research Project. A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes1993 · Cell · PMID 8458085Open reference}90346-1)). The protein is essential for normal embryonic development; complete knockout of HTT is embryonic lethal in mice by day E7.5 ^{4Trinucleotide repeat disorders2007 · Annual Review of Neuroscience · PMID 17417937Open reference}). In the adult brain, wild-type huntingtin plays critical roles in vesicle trafficking, transcriptional regulation, autophagymechanisms/autophagy), neurotrophic support, and synaptic function. A 2024 study showed that global HTT knockout in adult mice leads to fatal neurodegeneration, confirming that huntingtin remains essential throughout life [^11]). ^{1The Huntington's Disease Collaborative Research Project. A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes1993 · Cell · PMID 8458085Open reference}

Structure

Molecular Characteristics

Protein Domains and Architecture

Huntingtin has a complex multi-domain architecture organized around HEAT (Huntingtin, Elongation factor 3, protein phosphatase 2A, TOR1) repeat motifs that fold into superhelical solenoid structures ^{5Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain1997 · Science · PMID 9302293Open reference}): [^12]

1. N17 domain (residues 1-17): An amphipathic alpha-helix that is evolutionarily conserved and regulates membrane association, nuclear localization, and aggregation. Post-translational modifications (PTMs) in N17 profoundly alter toxicity and aggregation state ^{6Normal huntingtin function: an alternative approach to Huntington's disease2005 · Nature Reviews Neuroscience · PMID 16288287Open reference}).

2. Polyglutamine (polyQ) tract: The expanded CAG-encoded glutamine stretch that causes disease when >=36 repeats. The polyQ tract adopts beta-hairpin conformations in fibrillar aggregates, as revealed by cryo-EM ^{7The molecular biology of Huntington's disease2022 · Disorder Markers · PMID 36035060Open reference}).

3. Polyproline region: Located immediately C-terminal to the polyQ tract; modulates aggregation kinetics and protein-protein interactions.

4. HEAT repeat clusters: Multiple HEAT repeat regions spanning the protein form a solenoid scaffold that mediates diverse protein-protein interactions, including with HAP1, dynactin, and transcription factors.

5. Nuclear localization signal (NLS) and nuclear export signal (NES): Enable bidirectional nucleocytoplasmic shuttling.

Post-Translational Modifications

Huntingtin undergoes numerous PTMs that regulate its function, localization, and toxicity: [^13]

• Phosphorylation: Serine 13 and serine 16 in the N17 domain are protective; their phosphorylation reduces aggregation and toxicity. Threonine 3 (T3) phosphorylation is decreased in HD, and restoring it modulates mutant HTT conformation ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}0).

• SUMOylation: SUMO modification of lysines in the N17 domain increases aggregation and toxicity.

• Acetylation: Acetylation at K444 promotes autophagic clearance of mutant HTT.

• Palmitoylation: Palmitoylation by HIP14 regulates membrane association and vesicle trafficking.

• Proteolytic cleavage: Cleavage by caspases (particularly caspase-6 at D586), calpains, and other proteases generates N-terminal fragments containing the expanded polyQ that are highly toxic and aggregate-prone.

Normal Biological Functions

Vesicle Trafficking and Axonal Transport

Wild-type huntingtin serves as a molecular scaffold for intracellular transport, associating with vesicles and organelles along microtubules. It interacts with huntingtin-associated protein 1 (HAP1) and the dynactin complex to regulate both anterograde and retrograde transport of cargoes, including BDNF-containing vesicles, mitochondria, and endosomal compartments ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}1). This transport function is especially critical for maintaining the health of long-range projection neurons in the cortex and striatum. [^14]

Transcriptional Regulation

Huntingtin acts as a scaffold for transcription factors including RE1-silencing transcription factor (REST/NRSF), NCoR, CtBP, and p53, modulating expression of genes involved in neuronal survival, synaptic plasticity, and [bdnf production ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}2). Wild-type HTT sequesters REST in the cytoplasm, preventing it from silencing neuronal genes in the nucleus. [^15]

Anti-Apoptotic and Neuroprotective Roles

Normal huntingtin protects against apoptosis through multiple mechanisms: [^16]

• Inhibiting caspase-3 and caspase-9 activation

• Promoting mitochondrial integrity

• Modulating p53 function

• Protecting against excitotoxicity via regulation of nmda-receptor receptor] receptor] receptor] signaling

Neurotrophic Support

Huntingtin facilitates the transcription and axonal transport of brain-derived neurotrophic factor (BDNF), a critical survival factor for medium spiny neurons in the striatum. Wild-type HTT binds REST in the cytoplasm, promoting BDNF transcription; this function is disrupted in HD ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}3). [^17]

Autophagy and Proteostasis

Huntingtin plays a direct role in selective autophagy, interacting with p62-sqstm1, ULK1, and other autophagy machinery to facilitate clearance of damaged organelles and aggregated proteins [^16]). [^18]

Pathogenic Mechanisms in Huntington’s Disease

The mutant huntingtin protein (mHTT) with expanded polyQ tract (>=36 repeats) causes huntington-pathway through both toxic gain-of-function and partial loss of normal function: [^19]

Protein Aggregation and Phase Separation

mHTT forms intracellular aggregates (inclusion bodies) enriched in N-terminal fragments containing the expanded polyQ tract. Cryo-EM studies reveal that polyQ fibrils adopt a beta-hairpin core structure forming planar beta-sheets ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}4). Recent work highlights that HTT undergoes liquid-liquid phase separation (LLPS), and expanded polyQ disrupts this process, converting liquid condensates into solid aggregates [^15]). Aggregates sequester essential cellular proteins including ubiquitin, chaperones, and transcription factors.

Transcriptional Dysregulation

mHTT disrupts transcription through multiple mechanisms:

• REST/NRSF derepression: mHTT fails to sequester REST in the cytoplasm, leading to nuclear REST accumulation and silencing of neuronal genes including BDNF

• Sp1/TAFII130 interference: mHTT binds and sequesters transcription factors

• Chromatin remodeling: Altered histone-modifications and HDAC recruitment

• RNA Pol II impairment: Disrupted transcription elongation

Key affected pathways include BDNF expression, dopamine signaling, mitochondrial biogenesis, and neuronal survival ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}5).

Mitochondrial Dysfunction

mHTT impairs mitochondrial function through:

• Decreased PGC-1alpha expression, a master regulator of mitochondrial biogenesis

• Impaired mitochondrial-dynamics (fusion/fission balance)

• Reduced respiratory chain complex II and III activity

• Disrupted calcium handling and increased susceptibility to calcium-induced permeability transition

• Increased production of oxidative-stress

A 2024 study using human brain-organoids showed that mHTT disrupts CHCHD2-mediated mitochondrial metabolism during neurodevelopment, suggesting pathology begins far earlier than clinical onset [^12]).

Impaired Autophagy and Proteostasis

mHTT disrupts cellular [protein quality control]:

• Impairs cargo recognition in selective autophagy (autophagosomes form but are often empty)

• Disrupts lysosomal function

• Overwhelms the ubiquitin-proteasome-system

• Creates a vicious cycle of proteostatic stress as aggregates accumulate

Excitotoxicity

mHTT sensitizes neurons, particularly medium spiny neurons in the striatum, to excitotoxic cell death:

• Altered nmda-receptor receptor] receptor] receptor] subunit composition and trafficking (increased GluN2B surface expression)

• Increased intracellular calcium influx

• Activation of calpains and other calcium-dependent proteases

• Synergistic interaction with mitochondrial-dysfunction

Impaired BDNF Signaling

mHTT reduces neurotrophic support to striatal neurons:

• Decreased BDNF gene transcription (REST derepression)

• Impaired BDNF vesicle transport along corticostriatal axons

• Reduced TrkB receptor signaling in target neurons

This corticostriatal BDNF deficit is a major contributor to the selective vulnerability of medium spiny neurons in HD ^{2Huntingtin controls neurotrophic support and electrical activity of adenoviral brain-derived neurotrophic factor2004 · Cell · PMID 14744441Open reference}6).

Clinical Significance

CAG Repeat Length and Disease Onset

The length of the CAG repeat expansion is the primary determinant of disease onset and severity:

Age of onset correlates inversely with repeat length, though genetic modifiers (particularly DNA mismatch repair genes such as MSH3, PMS1, PMS2, and MLH1) also strongly influence onset [^14]). Somatic expansion of the CAG repeat, particularly in striatal neurons, is now recognized as a critical driver of disease progression.

Biomarkers and Disease Monitoring

• Mutant huntingtin (mHTT) in CSF: Detectable by ultrasensitive immunoassay; correlates with disease stage

• nfl-protein (NfL))): Elevated in CSF and plasma; tracks neurodegeneration

• neuroimaging: Striatal and cortical volume loss on MRI; emerging PET tracers for mHTT aggregates

• Digital biomarkers: Wearable sensor data capturing motor and cognitive decline

Therapeutic Approaches

Gene Silencing Strategies

The most promising therapeutic paradigm targets reduction of mHTT expression:

• Antisense oligonucleotides (ASOs): Tominersen (formerly IONIS-HTT_Rx/RG6042) was the first HTT-lowering ASO to enter phase III trials but was halted in 2021 due to unfavorable risk-benefit. Lessons learned are informing next-generation allele-selective ASOs that spare wild-type HTT [^13]).

• RNA interference (RNAi): AMT-130 (uniQure), a one-time AAV5-delivered miRNA targeting HTT, showed dose-dependent mHTT lowering in CSF and potential slowing of functional decline in phase I/II trials (Unidata from CHDI 2024 conference).

• CRISPR-based approaches: Emerging preclinical strategies for permanent inactivation of the expanded HTT allele.

Targeting Somatic Expansion

The discovery that DNA mismatch repair genes drive somatic CAG expansion has opened a new therapeutic axis. Inhibitors of MSH3 are in preclinical development to slow or prevent somatic repeat expansion in striatal neurons [^14]).

Symptomatic Treatments

1. Motor symptoms: Tetrabenazine and deutetrabenazine (VMAT2 inhibitors) for chorea; FDA-approved

2. Psychiatric symptoms: SSRIs/SNRIs for depression; atypical antipsychotics for psychosis and agitation

3. Cognitive symptoms: No approved treatments; [cholinesterase-inhibitors have limited efficacy

Emerging Therapies

• Small molecule splicing modulators: Branaplam (originally developed for SMA) was found to lower HTT but clinical development was halted

• Protein clearance enhancers: Strategies to boost autophagymechanisms/autophagy) and proteasomal degradation of mHTT

• Neuroprotective agents: Targeting mitochondrial function, excitotoxicity, and neuroinflammation

Brain Atlas Resources

• Allen Human Brain Atlas: Huntingtin Protein expression search

• Allen Mouse Brain Atlas: Huntingtin Protein search

• Allen Cell Type Atlas: Transcriptomic cell type reference

• BrainSpan Developmental Transcriptome: Huntingtin Protein developmental expression

See Also

• Clinical Trials Index

Background

The study of Huntingtin Protein (Htt) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying [mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

Conclusion

Huntingtin protein (HTT) is essential for normal neuronal development and function, with its mutation causing Huntington’s disease through a toxic gain-of-function mechanism. The expanded CAG repeat in the HTT gene leads to mutant huntingtin (mHTT) protein that forms aggregates, disrupts cellular transport, impairs mitochondrial function, and alters gene transcription.

Therapeutic strategies targeting HTT include:

• Gene silencing: ASOs and RNAi approaches to reduce mHTT expression

• Protein modulation: Enhancing autophagy and HTT clearance

• Function restoration: Targeting downstream pathways affected by mHTT

Recent clinical trials have focused on allele-selective ASOs that preferentially silence the mutant allele while sparing wild-type HTT, which is essential for normal cellular function. The challenge of delivering therapeutics to the striatum and [cortex, regions most affected in HD, remains an active area of research.

Understanding huntingtin’s normal functions in development, neuronal survival, and synaptic plasticity continues to inform therapeutic strategies. The goal of disease modification through HTT-lowering approaches represents the most advanced therapeutic pathway toward effective HD treatment.

External Links

• PubMed - Biomedical literature

• Alzheimer’s Disease Neuroimaging Initiative - Research data

• Allen Brain Atlas - Brain gene expression data