The use of forward genetics to analyze mammalian biology has been dramatically accelerated by methods that make it possible instantly to determine which mutation causes a phenotype. Now it is possible to discover gene function as rapidly as mutations can be created and screened: approximately 1,000 coding changes per week are interrogated in our laboratory. Moreover, it is possible to know approximately how much damage has been done to the genome over time. We estimate that we have damaged or destroyed about one-quarter of all protein encoding genes and tested the effects of variant alleles within these genes three times or more in a set of phenotypic assays that interest us. Only about two years were required to reach this level of saturation.