HHLA2 and osteosarcoma: Human endogenous retrovirus‐H Long repeat‐associating 2 (HHLA2, also known as B7H5/B7H7/B7y), as a new member of B7 family, was reported to be overexpressed in multiple human cancers, such as lung cancer, triple‐negative breast cancer, osteosarcoma, and renal cell cancer.6, 7, 8, 9, 10, 11