Knowledge regarding the molecular details of human cell growth stimulation by protooncogenes and growth suppression by tumor suppressor genes has facilitated molecular investigation into the mechanism of human cell senescence. Moreover, new pieces of information regarding the potential molecular basis of muscle differentiation have led to new hypotheses regarding one of the blockages in the cell cycle of the senescent cells. Pertinent to this proposal are the observations that the phosphorylation of the tumor suppressor protein, retinoblastoma p110 (Rb), in G1 phase of the cell cycle likely results in the loss of growth suppressor activity of the protein and, conversely, the dephosphorylation of the protein in G2/M phase of the cell cycle appears to restore the growth suppression activity of the Rb protein. We have shown that the apparent premature dephosphorylation of Rb in G1 phase by overexpression of type I phosphoprotein phosphatase results in cell cycle arrest, consistent with this role of the Rb protein. During muscle differentiation, we and others have hypothesized that the basic helix-loop-helix protein, MyoD, acts to maintain the Rb protein in an underphosphorylated state, thereby contributing to the growth arrest phenotype of the muscle cell. Senescent cells are growth arrested and contain only the hypophosphorylated form of the Rb protein. We hypothesize that of the multiple genetic components responsible for cellular senescence, at least one affects the phosphorylation state of Rb, and that by cDNA expression cloning combined with single cell assays of growth inhibition, genetic elements which cause senescence can be identified, including those involved in Rb regulation. For this we wish to 1) test the role of hypophosphorylated Rb protein in maintenance of the senescent phenotype, 2) employ cDNA expression cloning in single cell assay's of senescence to identify genes underlying the molecular basis of senescence, and 3) explore the mechanism of action of the genes identified in (2).