Members of the basic helix-loop-helix-PER-ARNT-SIM (bHLH-PAS) family are transcription factors that contain a bHLH DNA binding domain located amino-terminal to a PAS domain. Neuronal PAS domain protein 2 (NPAS2, also designated PAS 4/MOP4) is a member of the bHLH-PAS family and the PAS superfamily. NPAS2, which maps to chromosome 2p11.2-2q13, is expressed primarily in the neurons during the first week of postnatal development. The pattern of NPAS2 expression temporally matches the development of learning and memory, and spatially matches the frontal association/limbic forebrain pathway. NPAS2 may serve a regulatory role in the development and maintenance of long-term memory, and may be required for the processing of complex sensory information. NPAS2 and MOP3 form a transcriptionally active heterodimer which binds to a CACGTGA-containing DNA element and drives transcription from a linked luciferase reporter gene.