The tumor suppressor gene RASSF1A (3p21.3), human Ras association domain family 1A, is expressed in heart, brain, placenta, lung, liver, skeletal muscle, kidney, pancreas, spleen, thymus, prostate, testis, small intestine, colon, and peripheral blood leukocytes. RASSF1A expression is inhibited by hypermethylation of the CpG island including the promoter and early transcribed regions of the human RASSF1A gene. Hypermethylated RASSF1A promoter is observed in primary tumors. Furthermore, RASSF1A is related with cell cycle regulation, apoptosis and microtubule stability. Potential tumor suppressor. Isoform A interacts with CDC20, an activator of the anaphase-promoting complex, APC, resulting in the inhibition of APC activity and mitotic progression. More specifically, RASSF1a inhibits proliferation by negatively regulating cell cycle progression at the level of G1/S-phase transition by regulating accumulation of cyclin D1 protein. It also inhibits the activation of STK3 and STK4 and may play an additional role as a scaffold protein, directing these enzymes to sites of activation. Isoform C has been shown not to perform these roles, and no function has been identified for this isoform.