Evolution has preserved their basic biological functions. ras proteins of yeast can transform mammalian cells (37) and mammalian ras proteins can support the growth of mutant yeast cells (37, 38). Yet, ras proteins appear to ras GENES 781 participate in multiple, often distinct, biological processes. In mammals, they have been implicated in cellular proliferation (39) and terminal differentiation (40--42). In yeast, they can be required for survival (43, 44) or mating (45). Therefore, it is likely that ras proteins may function at a critical crossroads of signal transduction pathways. The study of ras genes and their oncogenic alleles is certainly a most fascinating field that has attracted scientists in terested in disciplines as diverse as yeast sporulation, control of cell prolifera tion, neural differentiation, carcinogen-DNA interactions, and human cancer. During the last couple of years, reviews dealing with the structure and biochemical properties of ras proteins (46) and with the involvement of ras genes in hUlman cancer (47-49) and in carcinogen-induced tumors (50) have been published. In this review, I have compiled current information on the different arleas of ras gene research in an attempt to convey an overall view of the fundamental role that ras genes play in normal and neoplastic cellular processes.