Class distribution of insertional hits in leukemic animals and healthy controls. (A) The overrepresentation of class 1 hits (established proto-oncogenes) in leukemias. Other signaling genes (class 2) are also more often hit than expected, in both leukemic and healthy mice. Class 3 summarizes hits in other loci. □ indicates leukemic mice; ▦, healthy mice; and ▪, expected value. The expected frequency of hits is calculated on the representation of the gene classes in the mouse genome, as derived from Futreal et al²¹  and Akagi et al³⁵  for class 1 and Figure 18 (gene ontology) of Waterston et al³⁶  for signaling genes. (B) Schematic representation of the leukemia-associated insertional class 1 hits (red) and class 2 hits (black) with respect to the promoter (P). Hits in reverse orientation are shown above the gene scheme, hits with forward orientation below. There is some preference for the 5-kb window upstream of the transcriptional start site, but not for a particular orientation.