CDK4 and neoplasm: In patient-derived tumor xenograft models, the response to ribociclib is closely correlated with retinoblastoma protein production.368 The combination of cetuximab and ribociclib showed no synergistic effect in the HNSCC model, but the administration of ribociclib promoted sensitivity to CDK4/6 inhibitors in cetuximab-resistant HPV-negative PDX models.359