Cbx1 gene function is essential, and its product, HP1-β, is a modifier of PEV. (A) The relevant regions in the wild-type Cbx1 locus (top; see Materials and methods), the TK-Neo^r targeting vector (middle), and the targeted gene (bottom). Coding regions are depicted by closed boxes. Noncoding regions are denoted by striped boxes. The DNA probe used to screen for targeting events is shown as a shaded rectangle. (B) Southern blot authentication of germline transmission of the Cbx1 mutation. BamHI digest of genomic DNA produces an 11.6-kb fragment for the wild-type (wt) allele and a diagnostic 5.3-kb fragment for the targeted allele. (C) Images of wild-type (left) and Cbx1^−/− (right) neonates. (D) Scatter plots showing the results of flow cytometry analysis of the proportion of transgenic embryonic DP (CD4⁺CD8⁺) thymocytes that express hCD2 taken from the three Cbx1 genotypes. Each point represents the result from a single embryo. The mean expression of hCD2 in embryonic thymocytes from Cbx1^+/− animals is higher than that for Cbx1^+/+ animals, but not significantly. The mean expression of hCD2 in thymocytes taken from Cbx1^−/− animals is significantly different from expression in thymocytes taken from the parental genotypes (P < 0.001). Black lines represent the means.