Scheme used to test the effects of folate deficiency in the early preleukemic phase of Friend virus disease. Two weeks before Friend virus infection (Day −14), mice were fed either folate-free diet (□) or control diet (). All mice were infected with Friend virus on Day 0. Folate deficient 1 group was switched to control diet after 4 weeks of folate-free diet (Day 14) and folate-deficient 2 group was switched to control diet after 8 weeks of folate-free diet (Day 42). The control group was fed the control diet throughout the entire course of the experiment. Mice were observed until they developed leukemia or they survived for 1 year.