This mutation was found as a "parasitic lethal" in tw73 and recovered from the haplotype by recombination. A full-length murine endogenous retrovirus (MuERV) is inserted into intron 7. This results in the expression of an aberrant mRNA that contains sequences derived from exons 1-7, but is missing sequences derived from exons 8-11. 400 bp of viral sequence replaced the missing sequences in the mRNA.