Distribution of Xpr1 variants and endogenous X/P-MLV env genes in the genus Mus. The 4 subgenera originated about 7.5 million years ago (MYA). Red arrows and brackets mark the distribution of the 5 functionally defined Xpr1 alleles among Mus species and strains. The house mouse species are indicated by a bracket, and the specific MLV ERV env types found in Mus are listed on the right. The tree is adapted from the synthetic trees developed by Guenet and others [63, 64, 211].