There are six members of the RASSF gene family, with RASSF1 being the best characterized. All six genes produce proteins that contain Ras Association (RA) domains that can interact directly with activated Ras in overexpression studies. Their role in mediating the biological effects of Ras remains under investigation. However, they seem to modulate some of the growth inhibitory responses mediated by Ras. Moreover, evidence is accumulating that RASSF family members may serve as tumor suppressors that succumb to inactivation during the evolution of the transformed phenotype. Thus, RASSF proteins may be described as effector/tumor suppressors, in contrast to traditional Ras effectors such as Raf and PI-3 kinase, which may be considered to be effector/oncoproteins.