The analysis of various genetic components in the standard inbred laboratory strains of mice shows that several taxonomic units of the complex speciesMus musculushave contributed to their genetic background. These laboratory lines are often taken as archetypes of the mammalian genome and since their genealogy is known for around 80 years they have been used to estimate mutation rates and various other evolutionary parameters. The knowledge of their origins is important if one is to avoid erroneous interpretations. Do they possess haplotypes that could have existed in natural populations ofM. musculus domesticusor are they the fruit of artificial recombinations between divergent genomes? We discuss this in the light of various genetic systems.