Recombinant inbred (RI) strains are formed from an outcross between two well-characterized inbred stains followed by at least 20 generations of inbreeding to create several new inbred lines whose genome is a mosaic of the parental genomes. The resulting RI strains are independent inbred strains with unique phenotypes due to the allelic mixture of the parental genes. The founder strains of the LEXF/FXLE recombinant inbred strain set are F344/Stm and LE/Stm (pictures below).

The LEXF/FXLE RI strains and their parental strains, F344/Stm and LE/Stm, were originally generated at the Saitama Cancer Center Research Institute by Shisa et al.. LE/Stm was derived from a closed Long Evans colony from the Ben May Laboratory for Cancer Research of the University of Chicago, and F344/Stm originated from F344/DuCrj (Charles River Japan). The strains were inbred at the Saitama Institute for more than 50 and 23 generations, respectively. The RI lines were generated in two phases: first, the LEXF strains were established followed by the FXLE strains. Several RI lines had substrains that branched out at the 7th to 11th generation after an attempt to fix the coat color. These sublines are indicated by alphabetical letters B – D following the strain number; e.g. LEXF8D.

RI strains are particularely useful for the analysis of many complex phenotypic traits. Once the fully inbred status has been achieved, the results of a single genotyping can be utilized in all future experiments. In subsequent projects only the phenotype of the RI strains need to be determined and can be linked to the stable genomic information. Furthermore, multiple individuals from each strain can be examined to reduce individual, environmental, and measurement variability, and multiple invasive phenotypes can be obtained on the same set of RI genomes, which is not possible in F2 or back cross progenies.

The LEXF/FXLE panel has been examined in the context of the NBRP-Rat phenome project for 74 quantitative parameters. Furthermore, micrisatellite marker (357) and SNP marker (~20.000) are determined for the whole strain set. The successful application of these marker sets to the from the phenom project obtained phenotypes has impressively shown the power of this strain tool set through the detection of more than 250 significant QTLs.
Summaries of the QTLs are available from our web site. Table 1 (Physiological Genomics, 32(3): 335-342, 2008), Table 2(Nature Genetics, 2008)