KRAS is a gene that provides instructions for making a protein called K-Ras, a part of the RAS/MAPK pathway. The protein relays signals from outside the cell to the cell's nucleus. These signals instruct the cell to grow and divide or to mature and take on specialized functions. It is called KRAS because it was first identified as a viral oncogene in the Kirsten RAt Sarcoma virus. The oncogene identified was derived from a cellular genome, so KRAS, when found in a cellular genome, is called a proto-oncogene. The K-Ras protein is a GTPase, a class of enzymes which convert the nucleotide guanosine triphosphate into guanosine diphosphate. In this way the K-Ras protein acts like a switch that is turned on and off by the GTP and GDP molecules. To transmit signals, it must be turned on by attaching to a molecule of GTP. The K-Ras protein is turned off when it converts the GTP to GDP. When the protein is bound to GDP, it does not relay signals to the nucleus.