<p>PAS domains are involved in many signalling proteins where theyare used as a signal sensor domain. PAS domains appear in archaea,bacteria and eukaryotes. Several PAS-domain proteins are known todetect their signal by way of an associated cofactor. Haeme,flavin, and a 4-hydroxycinnamyl chromophore are used in differentproteins. The PAS domain was named after three proteins that itoccurs in: </p><li>Per- period circadian protein</li><li>Arnt- Ah receptor nuclear translocator protein</li><li>Sim- single-minded protein.</li><p>PAS domains are often associated withPAC domains <db_xref db="INTERPRO" dbkey="IPR001610"/>. It appears that these domains are directly linked, and that together they form the conserved 3D PAS fold. The division between the PAS and PAC domains is caused by major differences in sequences in the region connecting these two motifs [<cite idref="PUB00014500"/>]. In human PAS kinase, this region has been shown to be very flexible, and adopts different conformations depending on the bound ligand [<cite idref="PUB00014501"/>].Probably the most surprising identification of a PAS domain was that inEAG-like K<sup>+</sup>-channels [<cite idref="PUB00005472"/>].</p>