A specific probe for detecting ecotropic murine leukemia virus sequences was constructed by cloning a 500-base-pair DNA segment, corresponding to a portion of the env region of the AKR ecotropic virus, in a pBR322/Escherichia coli K-12 host/vector system. This probe was used to screen the cellular DNAs of six inbred strains of mice for the presence of ecotropic retroviral DNA sequences by the Southern blot hybridization procedure. Three copies of ecotropic viral DNA were detected in AKR/N (a high-ecotropic virus strain) and two were found in BALB/c (a low-ecotropic virus strain) DNAs. As expected, no sequences reactive with this probe were found in NFS mouse DNA (a virus-negative strain). However, cellular DNA sequences that reacted strongly with the ecotropic-specific DNA probe were detected in certain NZB, C57L, and 129 mice (all virus-negative strains). In contrast to the reactive sequences in AKR and BALB/c, the reactions were chiefly associated with EcoRI segments that were subgenomic in size.

Chan, H. W., Bryan, T., Moore, J. L., Staal, S. P., Rowe, W. P., & Martin, M. A. (1980). Identification of ecotropic proviral sequences in inbred mouse strains with a cloned subgenomic DNA fragment. Proceedings of the National Academy of Sciences, 77(10), 5779–5783.