This chapter describes the Ras and Rap I interaction with AF-6 effector target. The chapter presents a spectrum of investigative approaches that served to demonstrate specific protein interactions among the Ras/Rapl GTPases and their potential effector molecule AF-6 in vivo and in vitro. The Rap types of small GTPases are members of the Ras superfamily and are the molecules that show the most identity with the oncogenic Ras proteins. Whereas the interaction of activated Ras proteins with their downstream effectors Raf, Ral guanine nucleotide dissociation stimulator (RalGDS), and phosphatidylinositol 3-kinase (PI3K) leads to a fairly defined picture, the role of Rapl is as yet poorly understood. The two-hybrid analysis and the investigation of the kinetic and thermodynamic properties of Ras-AF-6 and Rapl-AF-6 complexes leads to the same overall outcome—namely, Rap1 appears to form the tightest complex with AF-6-RBD1. The chapter concludes with a discussion on methods to quantitate Ras/Rapl and AF-6 interactions.