DAPI cell counts in the four additional RB cell lines Y-79, RB355, RBL-30 and RBL-15 confirmed the data gained in WERI-Rb1 cells, generalizing the notion of a pro-apoptotic effect of RA, BMP-4 and combined treatment in RB cell lines (S4 Fig). Here, BMP4 is linked to retinoblastoma.