Abstract: A novel conserved amino acid motif ("PAS") which provides a binding site for homo and hetero protein interactions has been found in mammalian and insect proteins. Abnormalities in these protein interactions are believed to be responsible for a variety of human diseases or conditions, including behavioral disorders and epithelial tissue cancers. Methods for identifying persons who have a disposition towards these behavioral disorders or epithelial tissue cancers are described. Methods are also described for identifying agonists and antagonists of proteins or related peptides containing PAS domains. Screening such agents involves assessing the ability of candidate compounds to promote or interfere with the binding of certain biologic preparations comprised of PAS-containing proteins. Successful agonists or antagonists should be useful in modifying the effects of human behavioral disorders, as well as certain epithelial cancers.