Flap structure-specific endonuclease 1 (FEN1) is a key enzyme in base excision repair (BER) and DNA replication [25,26], and it is related to the chemoresistance of multiple cancers such as breast cancer [27,28,29], non-small cell lung cancer (NSCLC) [30], and osteosarcoma [31]. The gene discussed is FEN1; the disease is osteosarcoma.