Identification of retrovirus CISs in myeloid leukemia of p15Ink4b-targeted mice. (A) Southern blot analysis of DNAs isolated from leukemic spleens of p15Ink4b-targeted (fl/fl and fl/wt), MOL4070LTR-infected mice. Genomic DNA for Southern blot analysis was isolated by phenol extraction, digested with EcoRI, separated on a 0.7% agarose gel, and transferred onto a nylon membrane. The blots were hybridized with a viral LTR probe labeled by the random priming method as described earlier.¹²  (B) Two CISs close to c-Myb and Sox4 encoding genes were identified in myeloid leukemias in p15INK4b-LysMcre mice by inverse and ligation-mediated PCR. (C) Northern blot analyses of c-myb expression in myeloid leukemias. (◁), aberrantly expressed c-myb mRNA in leukemias with retrovirus integrated within the gene. *Transcription in leukemia with retrovirus integrated 23 kb upstream of the c-myb gene. (D) Northern blot analyses of Sox4 expression in myeloid leukemias. *Leukemias with retrovirus integrated upstream or downstream of the Sox4 gene. Loading and integrity of analyzed RNAs were confirmed by hybridization with a gapd probe.