The Hart Lab studies essential genes in cells to learn about gene function and to identify genetic vulnerabilities in tumors. The DNA in every human cell encodes around 20,000 proteins. However, only about two thirds of those are expressed in a given cell, and only about two thirds of those are constitutively expressed in every cell. Not surprisingly, then, there are two broad classes of essential genes: "core fitness genes" are essential for cellular proliferation in every cell, and "context fitness genes" are required only in the presence of a particular oncogenic driver, genetic background, nutrient environment, or epigenetic state (i.e. tissue specific). Each of these classes provides unique opportunities for tumor-specific cancer targeting.