The KRAS gene provides instructions for making a protein called K-Ras 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 (proliferate) or to mature and take on specialized functions (differentiate) The K-Ras protein is a GTPase which means it converts a molecule called GTP into another molecule called GDP In this way the K-Ras protein acts like a switch that is turned on and off by the GTP and GDP molecules KRAS is usually tethered to cell membranes because of the presence of an isoprene group on its C-terminus There are two protein products of the KRAS gene in mammalian cells that result from the use of alternative exon 4 (exon 4A and 4B respectively) K-Ras4A and K-Ras4B these proteins have different structure in their C-terminal region and use different mechanisms to localize to cellular membranes including the plasma membrane