Laboratory mice are the same species as the house mouse, however, they are often very different in behaviour and physiology. There are hundreds of established inbred, outbred, and transgenic strains. A "strain", in reference to rodents, is a group in which all members are as nearly as possible genetically identical. In laboratory mice, this is accomplished through inbreeding. By having this type of population, it is possible to conduct experiments on the roles of genes, or conduct experiments that exclude genetic variation as a factor. In contrast, outbred populations are used when identical genotypes are unnecessary or a population with genetic variation is required, and are usually referred to as "stocks" rather than "strains". Over 400 standardized, inbred strains have been developed.