HS1BP3 is a 392 amino acid protein that contains one PX (phox homology) domain, a leucine zipper, immunoreceptor tyrosine-based inhibitory motif-like motifs and multiple proline-rich regions. Expressed primarily in brain, HS1BP3 is encoded by a gene mapping to human chromosome 2p24.1. The gene encoding HS1BP3 is frequently mutated in familial essential tremor, a disorder characterized by kinetic tremor the the hands, voice or head, though there is no correlation to Parkinson disease. HS1BP3 interacts with HAX-1’s SH3 domain, and may also play a role in the regulation of catecholamine and serotonin metabolism. Acting as a regulator of IL-2 signaling, HS1BP3 is likely involved in lymphocyte activation.