ATG3 (APG3) is a widely expressed conjugating enzyme for APG8 lipidation, an essential step for the initiation of autophagy. Autophagy, the process of bulk degradation of cellular proteins through an autophagosomic-lysosomal pathway is important for normal growth control and may be defective in tumor cells. Autophagy is involved in the preservation of cellular nutrients under starvation conditions as well as the normal turnover of cytosolic components. This process is negatively regulated by TOR (Target of rapamycin) through phosphorylation of autophagy protein APG1. ATG3 functions as an E2-like enzyme during the initial stages of autophagosome formation by catalyzing the formation of the Atg8-phosphatidylethanolamine (Atg8-PE) conjugate, which is critical for autophagy.