He took the Cold Spring Harbor course on animal virology in 1961 and he moved to Richard Franklin's lab at the Rockefeller Institute at New York City which was one of the few labs pioneering molecular research on animal virology. There he made fundamental discoveries on virus replication and its effect on cell metabolism, including the first description of an RNA replicase. He completed his PhD thesis work in 1964. After his PhD, Baltimore returned to MIT for postdoctoral research with James Darnell in 1963. He continued his work on virus replication using poliovirus and pursued training in enzymology with Jerard Hurwitz at Albert Einstein College of Medicine in 1964/1965. In February, 1965, Baltimore was recruited by Renato Dulbecco to the newly established Salk Institute for Biological Studies in La Jolla as an independent research associate. There he investigated poliovirus RNA replication and began a long and storied career of mentoring other scientists' early careers including Marc Girard, and Michael Jacobson. They discovered the mechanism of proteolytic cleavage of viral polyprotein precursors, pointing to the importance of proteolytic processing in the synthesis of eukaryotic proteins. He also met his future wife, Alice Huang, who began working with Baltimore at Salk in 1967. He and Alice together carried out key experiments on defective interfering particles and viral pseudo types. During this work, he made a key discovery that polio produced its viral proteins as a single large polyprotein that was subsequently processed into individual functional peptides. In 1968, he was recruited by soon-to-be Nobel laureate Salvador Luria to the Department of Biology at MIT as an Associate Professor of Microbiology. Alice S. Huang also moved to MIT to continue her research on vesicular stomatitis virus (VSV). They became a couple, and married in October 1968. At MIT, Huang, Baltimore, and graduate student Martha Stampfer discovered that VSV involved an RNA-dependent RNA polymerase within the virus particle, and used a novel replication strategy to replicate its RNA genome. VSV entered a host cell as a single negative strand of RNA, but brought with it RNA polymerase to stimulate the processes of transcription and replication of more RNA. Baltimore extended this work and examined two RNA tumor viruses, Rauscher murine leukemia virus and Rous sarcoma virus. He went on to discover reverse transcriptase (RTase or RT) - the enzyme that polymerizes DNA from an RNA template. In doing so, he discovered a distinct class of viruses, later name retroviruses, that use an RNA template to catalyze synthesis of viral DNA.