The generation and analysis of mutants has had an essential role in the identification of genes involved in cell cycle control (1 –3 ). In this regard, the genetic analysis of mutations has helped reveal the normal function of wild-type gene products as well as provided powerful insights into the intricate relationships between large numbers of gene products. The use of genetic screens and straightforward selection techniques in simple eukaryotes, like the yeastSaccharomyces cerevisiae, has greatly facilitated the ability to study and understand complex processes like cell cycle regulation (1 –3 ).
