RB1 and leiomyosarcoma: Interestingly, Rubio et al. showed that loss of TP53 and RB in osteogenic progenitors derived from murine BM-MSCs, but not ASCs, resulted in the formation of metastatic osteosarcoma.139 Additionally, leiomyosarcoma-like tumors were promoted in TP53 and RB null undifferentiated BM-MSCs or ASCs.139 Together these observations suggest that not only is a certain level of osteogenic differentiation required for osteosarcoma development but that the source of the cells of the osteogenic lineage is also important.