USP13 and cholangiocarcinoma: The binding was found to prevent c-Myc ubiquitination and enhance c-Myc activity, thereby, increasing c-Myc-mediated purine synthesis in CCA.124 In turn, a CLK3-USP13-c-Myc feedback loop was identified whereby activated c-Myc increased CLK3 transcription by enhancing CLK3 promoter activity.