Expression analysis of SRS19-6–tagged genes. Splenic tumor cDNA derived from (A) AML mice and (B) T-ALL mice was subjected to quantitative RT-PCR. Expressions levels were normalized to the level of β-actin mRNA. The expression level of each gene was further normalized to a sample consisting of pooled cDNA from 10 different tumors without retroviral integrations in the gene in question (; all genotypes). This sample was arbitrarily set to 1. All measurements were performed in triplicate, and standard deviations are depicted. The different genotypes are indicated by □ (Cebpa^+/+), (Cebpa^+/BRM2), or ■ (Cebpa^BRM2/BRM2). To determine the contribution of the tumor clone, containing an integration in a given gene, to the total tumor mass, we performed gene-specific southern blots. Splenic genomic DNA was isolated from WT (N) and diseased (T) animals and subjected to Southern blot analysis using gene-specific probes. *Bands representing RISs. See Figure S2 for a blow-up of the Southern blots.