  The term molecular genetics sometimes refers to a fundamental theory alleging that genes direct all life processes through the production of polypeptides, sometimes to a more modest basic theory about the expression and regulation of genes at the molecular level, and sometimes to an investigative approach applied throughout biomedical science that is based on investigative strategies grounded in the basic theory about genes. This article briefly summarizes the basic knowledge of molecular genetics and surveys philosophical research on the subject. The survey is organized around three questions that have dominated philosophical attention: (a) Can classical genetics be reduced to molecular genetics? (b) What is a gene? and (c) What do genes do? It concludes by turning attention to a fourth question: (d) Why is so much biological research centered on genes and DNA?