RB1 and breast carcinoma: To better investigate the drug-mediated induction of cell-cycle arrest, we performed a Western blot analysis of different proteins involved in cell-cycle progression control, such as retinoblastoma (Rb), p21, cyclins A, B, D, E. As shown in Fig 2B, nelfinavir reduced the levels of cyclin A, cyclin B, cyclin D, phosphorylated Rb, and increased the expression levels of p21 in breast cancer cell lines but not in normal cells.