Presumptively regressive infections (n = 28) had significantly higher anti-SU antibody ELISA levels (2.5/1.9 against FeLV-A/FeLV-B) than both unvaccinated/unexposed cats and presumptively progressively infected cats (p < 0.001 for both; Mann–Whitney U-tests). The gene discussed is ERVW-1; the disease is infection.