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Extracts of monkey cells (CV-1P) synthesizing SV40 large T antigen (T) were immunoprecipitated with monoclonal antibodies to T or pflO-114ab, the product of the retinoblastoma susceptibility gene (Rb). While a family of pIlO-lf4ab proteins can be detected in anti-Rb immunoprecipitates, only one member of this family, pllORb, was found in anti-T precipitates of these extracts. identical results were obtained with extracts of CV-1P cells which had been previously mixed in vitro with highly purified T. The pIlO-114abfamily is composed of two sets-pllORb, an un-or underphosphorylated species, and p~ 112-114~~, a group of overtly phosphorylated proteins. Thus, T bound preferentially to the un-or underphosphorylated member of the family. In addition, T failed to alter the relative abundances of these species. These results suggest a model in which the growth suppression function of Rb is down modulated either by …