MX1 and infection: Shin et al. (2015) showed that even the protectiveness of Mx1 from the A2G inbred strain is conditional and depends on host genetic background. Thus, CC-RIX designs that incorporate heterozygous classes of domesticus Mx1 crossed with either CAST Mx1 or musculus Mx1 can be of substantial benefit for mapping novel loci affecting infection outcomes, and at least 40% of the F1 crosses in our CC-RIX study incorporate lines which have one single copy (CAST or musculus) of Mx1.