A study detecting PD-1, PD-L1, and PD-L2 expression in 234 clinical samples from patients with musculoskeletal tumors reported that PD-1 and PD-L1 were negatively associated with prognosis and overall survival in osteosarcoma patients.19 This study also suggested that nivolumab, a PD-1 inhibitor, increased the proportions of CD4+ and CD8+ T lymphocytes and improved the cytotoxicity of CD8+ T lymphocytes to effectively suppress pulmonary metastasis of osteosarcoma in in vivo assays. This evidence concerns the gene CD4 and osteosarcoma.