MYC and carcinoma: pRb was hypophosphorylated to the active form in tylophorine-treated carcinoma cells, whereas its protein expression levels remained unchanged (Fig. 3B); this result was expected because the protein expressions of the pRb's upstream regulators, cyclins D1 [8, 9]/D2 [10, 11] and c-Myc 1, 7], were reduced by tylophorine treatment thereby decreasing the phosphorylation of pRb (Fig. 3B).