USP15 and liver cancer: Recently, XOR was found to aggravate the accumulation of ROS in liver cancer stem cells by promoting ubiquitin-specific peptidase 15 (USP15)-mediated nuclear factor erythroid 2-related factor 2 (Nrf2)-Kelch-like ECH associated protein 1 (KEAP1) signaling, ultimately block liver cancer stem cell and tumor propagation [45].