Ras protein or ras protein (r?s)n. Any of a group of proteins that are found near cell membranes and regulate cell division and proliferation. Abnormal Ras proteins facilitate uncontrolled cell division, leading to the development of tumors.[ra(t) s(arcoma), from the fact that genes coding for abnormal ras proteins were first isolated from rat sarcomas.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.