Neurexins comprise a family of neuronal cell surface proteins, which include neurexin I (NRXN1), neurexin II (NRXN2), neurexin III (NRXN3) and CASPR (neurexin IV). CASPR4 (contactin associated protein-like 4), also known as CNTNAP4, is a 1,308 amino acid single-pass type I membrane protein belonging to the neurexin family. Possibly a cell adhesion protein, CASPR4 is expressed in multiple regions of the brain such as olfactory bulb, hippocampus and deep cerebellar nuclei, with highest expression found in spinal cord, substantia nigra and subthalamic nucleus. CASPR4 is expressed at moderate levels in fetal brain, with low expression in adult testis. Encoded by a gene that maps to human chromosome 16q23.1, CASPR4 contains four laminin G-like domains, a F5/8 type C domain, two EGF-like domains and a single fibrinogen C-terminal domain.