to extend scientists' understanding of mammalian DNA replication. Herschel Mitchell and Giuseppe Attardi, their advisors, felt that the papers should be published under Huberman and Riggs' names, without their advisors', because they had done the work independently. Protein–DNA interaction Riggs moved to the Salk Institute to study protein–DNA interactions with Melvin Cohn as a postdoctoral fellow from 1966 to 1969. At the time, two proteins had been identified as binding to DNA and controlling gene expression: the lambda repressor and the lac repressor. Riggs worked on isolating the lac repressor by affinity chromatography. Walter Gilbert and Benno Müller-Hill were the first to successfully identify it, using a different technique. However, Arthur Riggs was the first to purify usable quantities of a transcription factor protein, the lac repressor. He and Suzanne Bourgeois developed a nitrocellulose filter binding assay method that was much faster than existing methods of analysis. Their work resulted in another well-known series of papers on the lac repressor and bacterial gene regulation, opening up new areas of research and theory. City of Hope National Medical Center As a result of his interest in gene regulation in mammalian cells, Riggs became curious about X chromosome inactivation, in which one of the two copies of the X chromosome present in a female mammal is inactivated. One of its co-discoverers, Susumu Ohno, worked at the City of Hope National Medical Center. In 1969 Riggs joined the department of molecular biology at the City of Hope National Medical Center as an associate research scientist. He became a senior researcher in 1974, Associate Chair of the Division of Biology in 1979, and Chair of the Division of Biology in 1981. Somatostatin and insulin Riggs continued to study the lac repressor and examined gene regulation in bacteria with Richard E. Dickerson, John Rosenberg, and Keiichi Itakura. They hoped to mix E. coli lac repressor with lac operator, crystallize it, and examine the protein–DNA binding of the crystals using high-resolution DNA methylation analysis. They were able to clone Itakura's lac operator and confirm that it would work in live bacteria, an important result. They also developed a method in which researchers add short "linkers" to DNA sequences and insert them into the bacterial genome. Riggs and Itakura collaborated with Herbert Boyer at Genentech, and used recombinant DNA technology to become the first to produce a human protein in E. coli. Following the advice of Riggs