GTPase Activating Protein (GAP) is involved in down-regulating normal ras proteins and in the signal transduction pathway of some growth factors. We have screened 188 human tumours for mutations in the catalytic domain and at the C terminal SH2 region GAP. Three nonsense mutations in basal cell carcinomas were detected in the SH2 region and no mutations could be demonstrated in the catalytic domain. We conclude that mutations in the SH2 region of GAP may play a role in tumorigenesis and that inactivating mutations of the GAP catalytic domain do not contribute to tumour development.