Ras proteins are now well recognized for their essential function in transducing extracellular signals that regulate cell growth, survival, and differentiation. Although ras genes were originally identified in the mid-1960s as the transforming elements of the Harvey and Kirsten strains of rat sarcoma viruses, investigation of the biological properties of their protein products did not gain momentum until the early 1980s, when mutated alleles of cellular ras genes were identified as dominant oncogenes in various types of human tumors. The mammalian ras gene family consists of three members: H-ras, K-ras, and N-ras, which are located on different chromosomes. The H-ras and K-ras genes are the cellular counterparts of the viral Harvey and Kirsten genes, respectively, and the N-ras gene is derived from a human neuroblastoma cell line. Are the different ras genes functionally redundant or does each ras gene have a specific role? This review brieffy recounts results from biochemical and genetic studies supporting the unanticipated possibility that the answer to both questions may be yes.