None of the mice vaccinated with dhfr-ts− CC1 rLm developed lesions despite multiple injections, consistent with prior studies where very high inocula were required (Titus et al., 1995), while all mice receiving WT CC1 vaccination developed significant swelling within 2 months, with some developing ulcerative lesions (although not to the degree seen with FV1 infections). The gene discussed is DHFR; the disease is infection.