A restriction endonuclease cleavage map of the genome of AKV, the endogenous, ecotropic leukemia virus of AKR mice, has been derived. By using this map and analyzing DNA from congenic mice, we have defined four DNA fragments diagnostic for AKV proviruses. Analysis of DNAs from 10 strains of American laboratory mice revealed that all strains carrying inducible, ecotropic murine leukemia viruses yielded DNAs which contained the four DNA fragments diagnostic for AKV. Virus-negative strains lacked these fragments in their DNA. Screening DNA from 23 additional mice revealed that, among these mice, only mice from Asia gave rise to the DNA fragments diagnostic of an AKV provirus. We conclude that all of the endogenous ecotropic murine leukemia proviruses in American laboratory mice are closely related since they share a common set of restriction endonuclease cleavage sites. These proviruses appear to derive from the East Asian ancestors of these mouse strains. Analysis of DNA from six selected mice with an additional restriction endonuclease showed that greater than 97% of the nucleotide sequences in each provirus are contigous and that these endogenous proviruses are indistinguishable from proviruses introduced by exogenous infection.

Sloan-Kettering Institute. These sublines were inbred by Stockert for 3 and 4 years, respectively (23). The breeding stocks for AKR/Ski derived from stocks maintained by E. Boyce at the Memorial Sloan-Kettering Institute, which were derived from the AKR/J subline in 1960 (L. Stockert, personal communication). The AKR/Rb stocks came from A. Leonard (12). The AKR/Cum mice were a gift of N. Rosenberg of Tufts University School of Medicine. The AKXL recombinant inbred strains were derived by brothersister inbreeding beginning with the F2 generation of the cross of AKR/J with C57L/J (25). The L.AKR-Akv4 congenic strain, previously designated L.AKR-Akv-2, was developed from the cross of AKR/J with C57L/J, by nine serial backcrosses to the C57L/J strain, with selection for those backcross mice that expressed AKV at 4 to 7 weeks of age in either spleens or tail biopsies. The provirus present in the congenic strain was identified as Akv-4 by the criterion that the provirus segregated independently of A BC ()I