Expression of cell cycle markers during growth arrest induced by TPA. RD cells were treated with 10-7 M TPA for the times indicated. (A) whole cell lysates from untreated (-) or TPA-treated cells (+) were separated on 12% SDS-PAGE and analysed by immunoblotting with specific antibodies for the proteins indicated. α-tubulin expression shows equal loading. pRb was analysed on a filter from 7% SDS-PAGE. Densitometric analysis of bands provided quantification expressed as the ratio of cell cycle protein amount versus the α-tubulin amount. (B) Northern blots from total RNA of untreated (-) and TPA-treated cells (+). The loading was tested by reprobing the same filters with GAPDH. Densitometric analysis of bands provided quantification as the ratio of the amount of mRNA cell cycle protein versus mRNA GAPDH amount. The data shown are representative of three independent experiments.