SIX1 and osteosarcoma: It has been reported that USP1 can bind to ID proteins [11, 31], RPS16 [32], KDM4A/SIX1 [33, 34] and KPNA2 [30] in osteosarcoma, gastric cancer, hepatocellular carcinoma, prostate cancer and breast cancer respectively and maintain the stability of these proteins, which suggests that the target of USP1 could be cancer type-specific.