Effect of retinoblastoma tumor suppressor gene expression on chemosensitivity of human osteosarcoma cell lines. We examined the effects of inactivation of the RB gene on chemosensitivity of human osteosarcoma cell lines, using the MTT assay and calculating the inhibition index. Although the human osteosarcoma cell lines HOS and MG63 have a wild-type RB gene, SaOS-2 and OSrb (established from retinoblastoma patient) have no active RB gene. We used these 4 cell lines in growth inhibition assays for doxorubicin, cisplatin and methotrexate, and assessed the chemosensitivity. In the growth inhibition assay for methotrexate, cell lines lacking an active RB gene were more resistant than cell lines with an active RB gene.