general status of XY expression, male fertility, and sex ratio are given. Phenotypes falling severely outside the wild-type range are in red. (B) Evolutionary relationships among some of the mouse species in the genus Mus, following [19]. The branches are not to scale and not all species of Mus are shown. Most standard laboratory strains, including those used by Cocquet et al. [7], are derived from M. m. domesticus. Inferred copy numbers for Slx (orange) and Sly (blue) [13] are given for each lineage. Slxl1 is not shown. doi:10.1371/journal.pgen.1002955.g001 Slxl1I may be the cause of this HMS because copy number differences between the subspecies will yield hybrid males that are Sly deficient [7]. 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