A large insertion in the first intron of the SWR allele is attributed to a 5542 bp ETnII-b endogenous retrovirus (ERV). A strain survey indicated that this insertion occurs only in SWR of the strains tested. Tested strains were: SWR, C57BL/10J, C57L/J, C57BL/6J, C58/J, C57BR/cdJ, C57BLKS/J, FVB/NJ, 129S6/SvEvTac, CBA/J, DBA/2J, C3H/HeJ, BALB/cJ, AKR/J.