Strategy for collection of leukemia with intact or deficient Nf1 gene for this study. Nf1^+/+. MuLV indicates MuLV-infected Nf1-intact leukemia; Nf1^−/+.MuLV, MuLV-infected leukemia with engineered null Nf1 allele. Nf1^Evi2.MuLV, MuLV-infected leukemia with Nf1 disruption by proviral insertion into Nf1. * indicates that the leukemias were produced previously by the laboratory of Dr Camy I. Brannan (deceased) through multiple rounds of backcrossing Nf1-null mice onto the BXH-2 strain of mice.^12,29  PIS indicates proviral insertion site; and CIS, common proviral insertion site.