The Per-Arnt-Sim (PAS) domain is a 270 amino acid motif that mediates associations among various PAS family transcription factors. The PAS family contains neuronal specific transcription factors known as NPAS1, NPAS2 and NPAS3, which are involved the development and maintenance of learning and memory pathways. NPAS1 regulates erythropoietin expression in developing brain. NPAS2, also designated PAS 4/MOP4, associates with MOP3 to activate transcription. NPAS3, which localizes to the nucleus and is ubiquitously expressed in the adult brain, may be involved in neurogenesis and may control regulatory pathways relevant to psychotic illness and to schizophrenia. It regulates tracheal cell fates in the embryo and is necessary for the development of the posterior spiracles and the salivary gland duct. NPAS3 contains 1 basic helix-loop-helix (bHLH) domain, 1 PAC (PAS-associated C-terminal) domain, and 2 PAS (PER-ARNT-SIM) domains. Efficient DNA binding by NPAS2 requires dimerization with another bHLH protein.