The action of lipid phosphatases on inositol phosphates is thought to be one method that the cell uses to attenuate growth factor- or cytokine-induced mobilization of calcium. We have cloned a cDNA from a mouse spleen library that is virtually identical to the recently described inositol tetraphosphate and phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase, SHIP. Chromosomal localization studies in human and mouse by FISH mapping and interspecies backcrossing in mice have demonstrated that human SHIP is localized to 2q36–q37.1 and that mouse SHIP is localized to 1C5.