RB1 and osteosarcoma: S-phase arrest has been observed in mammalian cells with prolonged arrest at the G1/S boundary [26]; Rb(+/+) mouse embryo fibroblasts treated with cisplatin, etoposide or mitomycin [27]; human melanocytes treated with thymidine dinucleotides [28]; and human osteosarcoma cells transduced with the p21 gene [29].