Identification of retroviral insertions clustered in the 3′ untranslated region of Sox4. Shown is a schematic of the Sox4 locus, containing the retroviral insertions identified in the AKXD-23 tumor DNA samples. The black line represents a 19.3-kb XbaI fragment containing the Sox4 gene with additional enzyme sites shown below. The single exon encoding the SOX4 protein is depicted by the large gray rectangle. The size of the 5′ and 3′ untranslated regions, represented by thick gray lines, was estimated based on the partial Sox4 mRNA sequence (GenBank) and the reported size of Sox4 mRNA.⁵³  The MuLV insertions identified by SacII digestion and inverse PCR are shown as thin vertical lines. The numbers and arrows above each insertion indicate the tumor number from which it was identified and the orientation of the provirus relative to Sox4 transcription. The probe used to detect Sox4 in both Southern and Northern blots analyses is depicted by the dashed line.