Several mapping studies of families with multiple individuals who have bipolar affective disorder (BPAD) have demonstrated possible linkage of the trait to the pericentric region of chromosome 18 (18cen). Currently, the large size of the critical interval defined by these studies makes effective selection of candidate genes formidable. However, documentation of 18cen-linked families in which a parent-of-origin effect was observed in the transmission of the BPAD trait provides a clue to the nature of the putative gene; it may be imprinted. In the present study, we cloned IMPACT, the human homolog of the mouse imprinted gene Impact and mapped it to 18cen within the critical interval for BPAD. Human IMPACT encodes a protein with 320 amino acids and is expressed at high levels in the brain. Since only a small number of imprinted genes are estimated to be present in the entire genome, very few imprinted genes would be expected to be present in this particular chromosomal region. Hence, IMPACT represents a candidate gene for BPAD susceptibility. Alternatively, other as yet unknown imprinted gene(s) adjacent to IMPACT could contribute to the BPAD trait, since multiple imprinted genes may occasionally form clusters. Localization of human IMPACT at 18cen in this study defines a promising target region in which to search for putative BPAD genes.