Huntingtin (HTT, UniProt/Swiss-Prot entry P42858) may play a role in microtubule-mediated transport or vesicle function and is required for normal development. It is widely expressed as 2 alternatively polyadenylated transcripts, the larger one being predominantly present in adult and fetal brain. The HTT gene is linked to Huntington’s disease, a neurodegenerative disorder characterized by loss of striatal neurons, which is thought to be caused by an expanded, unstable trinucleotide repeat. Whether this leads to loss of function or gain of function is not clear. The expanded repeat may increase the affinity of the HTT protein for the huntingtin-associated protein-1, which is highly expressed in brain and seems to inhibit HTT expression.