From NeuroLex

Recombinant inbred mouse strain

• Externally Sourced Definition: Recombinant inbred (RI) strains contain unique, approximately equal proportions of genetic contributions from two original progenitor inbred strains. Traditionally, recombinant inbred (RI) strains are formed by crossing animals of two inbred strains, followed by 20 or more consecutive generations of brother x sister matings (Bailey 1971, Taylor 1978). Alternate breeding schemes can be used, such as creating RI strain sets from Advanced Intercross Lines, where F2 animals are nonsib mated for several generations, followed ultimately by 20 or more consecutive generations of brother x sister matings. Note that if backcrossing to one of the parental strains is involved, this will create recombinant congenic strains and should be named accordingly. RI strains should be designated by uppercase one- or two-letter abbreviations of both parental strain names, with the female strain written first, and separated by an uppercase letter X with no intervening spaces. All members of RI sets involving the same two strains will be serially numbered regardless of whether they were created in one or more laboratories. (http://www.informatics.jax.org/mgihome/nomen/strains.shtml)

• Definition Source: MGI_defSource