While human genomes are highly varied and usually do not show extensive homozygosity, the genomes of mice from an inbred strain are nearly devoid of genetic variation, and do show near-homozygosity at all loci. This creates an ideal situation for geneticists eager to probe the essential function of genes. Mutations introduced by a random germline mutagen can be tested for their ability to cause phenotypic change, and when such a change is present, the causative mutation can be mapped in real time. Using this approach, we have identified thousands of mutations that cause phenotype. At times, we seek to create new derangements of immunity, development, behavior, or cardiovascular function. Equally important, we are able to suppress diseases in the mouse. As saturation is approached, all genes with non-redundant function in a particular biological phenomenon may be identified, leading to breakthrough understandings in biology.