RAS proteins are small GTPases that act as molecular switches to transduce signals from activated receptors. When in its GTP-bound state, RAS can bind to and activate a range of downstream effector proteins, which may then result in diverse cellular outcomes like cell proliferation, survival, differentiation, and neoplastic transformation. Three RASgenes code for four highly homologous RAS proteins, NRAS, HRAS, and KRAS4B/KRAS4A. These proteins have identical effector binding domains and hence can interact with the same set of downstream effectors. However, RASisoforms have been shown to differ in their abilities to activate various downstream proteins. Mutations affecting the three prototype Ras oncoproteins, HRAS, NRAS and KRAS, show a high degree of tumor-type specificity. Oncogenic versions of HRAS are better than NRAS or KRAS at transforming fibroblast cells, whereas NRAS is better at transforming hematopoietic cells. Nearly 30% of human cancers, including solid tumors and hematologic malignancies, are associated with mutations in RAS genes. Therapies that target the RAS proteins and the signaling pathways that they control would therefore be very valuable in treating tumours that have activating RAS mutations. However, their potential might be even greater, as many tumours that lack RAS mutations have found other ways to activate the same pathways.