Geneticists have been using model organisms ranging from the house mouse to the single-cell bakers’ yeast, Saccharomyces cerevisiae, to study basic biological processes that regulate human development and physiology, and that can be compromised in various diseases. This has been possible because many of the genes that control these processes in humans are also present with similar functions in those other species; and because genes in model organisms can be mutated and deleted in the laboratory at will. Thus far, however, even in easy-to-manipulate yeast, genes had to be deleted one-gene at a time, often with additional undesired sequence modifications left behind in their genome.