Mark Ptashne (born June 5, 1940 in Chicago) is a molecular biologist and violinist. He currently holds the Ludwig Chair of Molecular Biology at Memorial Sloan–Kettering Cancer Center in New York. He was the first scientist to demonstrate specific binding between protein and DNA, and his lifelong work has been the elucidation of the molecular mechanisms of switch between lytic and lysogenic lifecyle of bacteriophage lambda, as well as how the yeast transcriptional activator Gal4 works.