The mouse is the most commonly used experimental animal, probably because of its small size and fecundity. A mouse can produce a litter every 3 weeks and 5 generations can be achieved in one year. There are many inbred and outbred strains in a variety of coat colours; these include leukaemic, spontaneous tumour bearing, obese, blind, tail-less and athymic nude. The majority of mice are used for cancer research.