Kaplan-Meier survival curves of treated and untreated B6C3F1 PR⁺ and F₂ mice. Untreated mice in the resistant parental background (B6C3F1 PR⁺) develop leukemia after a long latency and with low penetrance (▴). ENU treatment of PR⁺ mice in the B6C3F1 background increases leukemia penetrance and decreases latency (▾). Intercrossing SWR/J alleles has an effect equipotent to ENU treatment in B6C3F1 mice (▾ vs ■, P = .34; ▴ vs ▾ or ▴ vs ■, P < .001). Treatment with ENU further increases the rate and the incidence of leukemogenesis in F₂ mice (■ vs ●, P < .001).