Changes in levels of the retinoblastoma protein and its degree of phosphorylation on induction of differentiation. Indicated U-937 clones (antisense Rb clone A1 and mock clone M6) were induced with ATRA (1 μmol/L) or Vit D3 (0.1 μmol/L). At 0 and at 24 hours, cells were harvested and subjected to protein extraction followed by specific immunoprecipitation with an antiretinoblastoma protein antibody, 7% SDS-PAGE, and Western blot analysis, as described in the Materials and Methods. Molecular weights are indicated to the left (in kilodaltons). Immunoprecipitated pRb of different molecular weights are indicated with arrows (ppRb, hyperphosphorylated; pRb, hypophosphorylated).