ABCB1 and osteosarcoma: Similar behavior has been observed with other in-vivo models of metastatic spread by direct intravenous injections of doxorubicin-resistant osteosarcoma U2OS and Saos-2 variants (MDR1 overexpression by gene amplification) in athymic nude mice, when resistant cells were injected straight after in-vitro treatment, but not when cultured in a drug-free medium for a week before injection [44].