Diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) hydrolase is the enzyme responsible for maintaining the intracellular level of the dinucleotide Ap4A, the function of which has yet to be established. The APAH1 gene encoding this Ap4A hydrolase has been mapped by fluorescence in situ hybridization and PCR to human chromosome 9p13. Radiation hybrid panel mapping further located APAH1 between the IL11RA and the GALT genes, thus excluding it as a candidate gene for cartilage-hair hypoplasia, which maps proximal to GALT. Several tumor suppressor genes have previously been mapped within the 9p13-p21 region. Given that the FHIT gene at 3p14.2, which encodes a diadenosine 5',5"'-P1,P3-triphosphate (AP3A) hydrolase, is a candidate tumor suppressor, APAH1 should also be considered a potential tumor suppressor.