USP15 and neoplasm: Human ubiquitin-specific protease 7 (USP7) and ubiquitin-specific protease 15 (USP15) both regulate several biological processes and tumorigenesis, including DNA damage response, transcription, epigenetic control of gene expression, immune response, cell proliferation, apoptosis, autophagy, cell cycle, cell invasion, tumor genome integrity, and transcription regulation [20,21].