Abstract: Described herein is a method of modulating sumoylation of a Ras protein by small ubiquitin-like modifier (SUMO) proteins. Provided herein is a method for regulating the activity of a Ras protein. Also provided is a treatment of proliferative diseases, such as cancer, by introducing specific mutations to a mutant Ras protein that is associated with the proliferative disease. Described herein is a modified Ras protein. Described herein are recombinant vectors, cells comprising the vectors expressing the modified Ras proteins. Provided herein is an antibody that specifically binds to a sumoylated Ras protein. Described herein is a method for identifying an agent that interferes with the sumoylation of a Ras protein. Also described herein are therapeutic and prophylactic compositions. Also provided herein is a method of using a modified Ras protein to replace an endogenous mutant Ras protein that is associated with a proliferative disease.