CDK4 and soft tissue sarcoma: Interestingly, previous studies showed that palbociclib was considerably less active in a number of non-synovial soft tissue sarcoma cell lines (IC50 values of 8.95–26.63 μM), suggesting that an aberrant cyclin D1-CDK4/6-Rb axis is a specific feature for the SyS subtype of soft tissue sarcomas.26 One SyS cell line had a noticeably higher IC50 (CME-1), probably explained by the immortalization with SV40 large T antigen.