XL281 (BMS-908662) is a novel, potent and orally bioavailable small molecule Raf kinase inhibitor with potential antineoplastic activity. XL281 specifically inhibits RAF kinases, located downstream from RAS in the RAS/RAF/MEK/ERK kinase signaling pathway, which may result in reduced proliferation of tumor cells. RAS mutations may result in constitutive activation of the RAS/RAF/MEK/ERK kinase signaling pathway, and have been found to occur frequently in human tumors.