Human GAP1-InsP4 BP, also designated Ras p21 protein activator (GTPase-activating protein) 3 [Ins(1,3,4,5)P4-binding protein], is an 829-amino acid protein that binds phospholipids in both a calcium-dependent and -independent manner. GAP1, one of the Ras GTPase-activating protein families, comprises four distinct genes, including GAP1(m), GAP1-InsP4 BP, MRASAL (murine Ras GTPase-activating-like) and KIAA0538. This family contains an N-terminal tandem C2 domain, a GAP-related domain and a C-terminal pleckstrin homology (PH) domain. The PH domains of GAP1-InsP4 BP are essential for membrane targeting via binding of specific phospholipids. Following agonist-stimulated PtdIns(3,4,5)P(3) production, group I family PH domain containing proteins like GAP1-InsP4 BP specifically bind inositol phosphates, which are subsequently targeted to the plasma membrane.