Abstract
Recombinant DNA clones of a viral clone of spleen necrosis virus, an avian retrovirus, have long terminal repeats of different sizes. The variation was in the U3 region of the long terminal repeats and any one clone had U3 of the same size in both long terminal repeats. The U3 regions in the 5'' and 3'' long terminal repeat were shown to be derived from the 3'' long terminal repeat of parental virus DNA.