PTEN and melanoma: Moreover, these studies pointed out to mutations affecting the biological function of proteins, such as PREX2 (phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 2)-a PTEN-interacting protein and negative regulator of PTEN which is mutated in 14% [55], PPP6C encoding a serine/threonine phosphatase mutated in 9–12.4% and RAC1, encoding a GTPase of the RAS superfamily of small GTP-binding proteins mutated in 5–9.2% [56, 57] of sun-exposed melanoma cases.