Within the broad susceptibility region for bipolar disorder on the pericentromeric portion of chromosome 18, the highest allele sharing in our 22-pedigree series has been found in markers mapping to 18p11.2. Studies by other investigators on independently ascertained pedigrees have also shown increased sharing in this region, making 18p11.2 a plausible site for a candidate gene search. We found expressed sequence tags (ESTs) mapping within this area that are homologous to the myo-inositol-1-phosphate phosphohydrolase (myo-inositol monophosphatase: IMP) gene of Xenopus laevis. Since IMP has been proposed to be the potential target of lithium, a drug commonly used for the treatment of bipolar disorder, we proceeded to characterize the cognate transcript. Northern blot analysis detected a major transcript of 1.5 kb with abundant expression in adult and fetal tissues, but minimal expression in whole brain. In subcortical brain regions, however, substantial levels of transcript were evident, most prominently in the caudate. We have isolated and sequenced the full-length cDNA. The deduced amino acid sequence revealed approximately 54% identity with an existing human IMP, which we found mapped to chromosome 8, and IMP of other species. The sequence also included motifs characteristic of the IMP gene family. To provide a more precise location of this gene, mapping with a panel of radiation hybrids (RH) was conducted. Multipoint RH analysis placed the gene between GNAL and D18S71 within the 18p11.2 region. We, therefore, designated this novel gene as IMP.18p. The physical position and possible function suggest that IMP.18p is an important candidate gene for bipolar disorder.