STUB1 and hepatocellular carcinoma: To determine whether E3 ubiquitin ligases are involved in the regulation of EGFR ubiquitination by PSMD9, we next examined the expression of the ubiquitin ligases of EGFR (STUB1, Parkin and c-Cbl) in PSMD9-overexpressing and PSMD9-knockdown HCC cells [17–19].