Understanding the genetic basis of complex traits has become one of the major issues in genetics, but recent advances in this field are still faced with a difficulty, the so-called "missing heritability." It is speculated that missing heritability mainly stems from a large number of variants of small effect that are poorly detected by currently available methods. In order to overcome this problem, many recent genetic studies of complex traits have actively used outbred stocks of mice. However, most of the available outbred stocks have a limited amount of genetic variation, because many stocks originate from Swiss mouse colonies. We have repeatedly shown that wild-derived strains are a useful mouse resource since there is a large genetic diversity among these strains. Here, we give an overview of mouse resources produced by crossing different founder mice. Finally, we propose an advantage of new attempts to conduct selective breeding using heterogeneous stocks created by mixing genomes of wild-derived inbred strains of mice when studying complex traits.