<p>The PAS (Per, Arnt, Sim) domain [<cite idref="PUB00006196"/>, <cite idref="PUB00005472"/>] is an approximately 300 amino-acid segment of sequence similarity which is conserved between the Drosophila protein period clock (PER), the Ah receptor nuclear translocator (ARNT) and the Drosophila single- minded (SIM). It is composed of two or more imperfect repeats (PAS-1, PAS-2). In addition, some proteins have another similar region of 40-45 amino acids situated carboxy-terminal to any PAS repeat and which contributes to the PAS structural domain: the PAC motif. The PAS family can be divided in two groups; the proteins that have the PAS motif followed by a PAC motif, and those that don't. Within the bHLH/PAS proteins, the PAS domain is involved in protein dimerization with another protein of the family. It has also been associated with light reception, light regulation and circadian rhythm regulators (clock).</p><p>In bacteria, the PAS domain is usually associated with the input domain of a histidine kinase, or a sensor protein that regulates a histidine kinase. </p>