““Working independently, Baltimore and Temin discovered an enzyme found in retroviruses that could build DNA from an RNA template. They called the enzyme reverse transcriptase-"reverse" because it inverted the normal direction of information flow: from RNA back to DNA, or from a gene's message backward to a gene, thereby violating Crick's "central dogma" (that genetic information only moved from genes to messages, but never backward). Using reverse transcriptase, ever RNA in a cell could be used as a template to build its corresponding gene. A biologist could thus generate a catalog, or "library" of all "active" genes in a cell-akin to a library of books grouped by subject. There would be a library of genes for T cells and another for red blood cells, a library for neurons in the retina, for insulin-secreting cells of the pancreas, and so forth. By comparing libraries derived from two cells-a T cell and a pancreas cell, say-an immunologist could fish out genes that were active in one cell and not the other (e.g., insulin or the T cell receptor). Once identified, that gene could be amplified a millionfold in bacteria. The gene could be isolated and sequenced, its RNA and protein sequence determined, its regulatory regions identified; it could be mutated an inserted into a different cell to decipher the gene's structure and function. In 1984 this technique was deployed to clone the T cell receptor-a landmark achievement in immunology.””
