Production of the retinoblastoma protein and changes in its degree of phosphorylation on induction of differentiation in Rb-transfected clones. Indicated U-937 clones (RB transfectants B3 and B4 and mock-transfectant M2) were induced with Vit D3 (0.1 μmol/L). At 0 and 24 hours, cells were harvested and indicated clones were biosynthetically labeled for 2 hours with ³⁵S-methionine/cysteine. Labeled proteins were subjected to immunoprecipitation with an anti-pRb antibody followed by 7% SDS-PAGE and fluorography, as described in the Materials and Methods. Immunoprecipitated pRb of different molecular weights are indicated with arrows (ppRb, hyperphosphorylated; pRb, hypophosphorylated).