RB1 and breast cancer: Soprano et al. reported slight accumulation of breast cancer cells in the G1 phase following treatment with nelfinavir for 24 h, associated with a clear reduction in cell cycle regulatory proteins cyclin D, E, A, B and phospho-Rb, and with an increase in the cell cycle inhibitory protein p21; strikingly, the cell cycle regulatory effects of nelfinavir observed in breast cancer cell lines were not evident in healthy breast epithelial cells [30].