Beclin 1 is the first identified mammalian gene to mediate autophagy and also has tumor suppressor and antiviral function. Autophagy, a process of bulk protein degradation through an autophagosomic-lysosomal pathway, is important for differentiation, survival during nutrient deprivation, and normal growth control, and is often defective in tumor cells. Beclin 1 was originally isolated in a yeast two hybrid screen to identify Bcl-2-binding partners and maps to a tumor susceptibility locus on human chromosome 17q21 that is frequently monoallelically deleted in human breast, ovarian and prostate cancer. Beclin 1 encodes an evolutionarily conserved 60 kDa coiled coil protein that is expressed in human muscle, epithelial cells and neurons. In gene transfer studies, beclin 1 promotes nutrient deprivation-induced autophagy, inhibits mammary tumorigenesis, and inhibits viral replication. Expression of the Beclin 1 protein is frequently decreased in malignant breast epithelial cells. Based upon these observations, it is speculated that beclin 1 may work through induction of autophagy to negatively regulate tumorigenesis and to control viral infections. Beclin 1 may also play a role in other biological processes in which autophagy is important such as cell differentiation and nutritional stress responses.