Table 1 Summary of unexpected conclusions deduced from the BLV/sheep model

Fusion-deficient viruses can propagate at wild type levels

A transformation-deficient tax mutant is leukemogenic in sheep

A large region between the env and R3 genes is dispensable for infectivity and viral spread

Deletion of R3/G4 affects but does not abrogate pathogenicity

Increasing LTR promoter activity decreases viral spread