IGF1R and osteosarcoma: Moreover, the ratio of IGF-1R:IR-A in favor of IR-A seems to be responsible of native and acquired resistance of some Ewing sarcoma to both monoclonal antibodies and small tyrosine kinase inhibitors (TKI) anti-IGF-1R and it may also explain the lower levels of sensitivity of other sarcomas, such as rhabdomyosarcoma and osteosarcoma to these targeted therapies.