Experiments with cloned complete tumor-derived bovine leukemia virus information prove that the virus is totally exogenous to its target animal species.

Taking advantage of the existence of a unique SacI restriction site in the long terminal repeats of the integrated bovine leukemia virus proviral DNA isolated from a bovine tumor, the total viral information (about 9.2 kilobases) was cloned in the lambdoid vector lambda WES. lambda B. Use of this cloned bovine leukemia virus DNA allowed us, for the first time, to definitely rule out the existence of any endogenous bovine leukemia virus sequence in the bovine, ovine, caprine, murine, feline, chicken, or human genomes. These data prove the absence of acquired cellular information in the provirus that has given rise to a tumor.