Lithium remains the most widely used long-term treatment for bipolar affective disorder, but the molecular mechanisms underlying its therapeutic efficacy have not been fully elucidated. Two enzymes involved in the phospholipase C signalling system, namely the myo-inositol monophosphatase (IMPase) and the inositol polyphosphate 1-phosphatase (IPPase), have been postulated as targets for the therapeutic action of lithium in manic-depressive illness. Intriguingly, Drosophila mutants lacking IPPase activity display a defect in synaptic transmission, and this alteration could be phenocopied by lithium exposure. We recently demonstrated the presence of several polymorphisms in the IPPase-encoding inositol polyphosphate 1-phosphatase gene (INPP1) cDNA and suggested that polymorphic variants of the human IPPase might be associated with the striking difference in lithium response among bipolar patients. We report the genomic structure and organization of the INPP1 gene on chromosome 2q32. Based on DNA sequencing of the entire genomic region containing INPP1, we found that the gene consists of six exons and spans more than 25 kb. Expression analysis showed that INPP1 is present as a 1.9 kb mRNA transcript in all organs and tissues examined, including the central nervous system. The level of expression varies, with at least a fourfold higher transcript level in testis compared with other tissues with high expression. A highly polymorphic dinucleotide repeat, (CA)18-25, with an observed heterozygosity of 0.86 was detected immediately downstream of the gene. The present sequence information will be used to further investigate the possible role of the INPP1 gene in lithium-treated bipolar illness.