INOSITOL 1,3,4,5-tetrakisphosphate (Ins(l,3,4,5)P₄) is produced rapidly from inositol 1,4,5-trisphosphate (Ins(l,4,5)P₃) in stimulated cells^1,2. Despite extensive experimentation, no clearly defined cellular function has yet been described for this inositol phosphate. Binding sites specific for Ins(l,3,4,5)P₄ have been identified in several tissues^3,4, and we have purified one such protein to homogeneity⁵. Its high affinity for Ins(l,3,4,5)P₄, and its exquisite specificity for this isomeric configuration^5,6, suggest it may be an Ins(l,3,4,5)P₄ receptor. Here we report the cloning and characterization of this protein as a GTPase-activating protein, specifically a member of the GAP1 family. In vitro it shows GAP activity against both Rap and Ras, but only the Ras GAP activity is inhibited by phospholipids and is specifically stimulated by Ins(l,3,4,5)P₄.