Targeting of the Cbx3 locus with conditional vector. Row 1, hnrnpA2B1/Cbx3 genomic locus in the mouse. The hnrnpA2B1 and Cbx3 genes are divergently transcribed and share a 3 kb CpG island (green). The ATG start sites are indicated by bent arrows; the alternative start sites for the Cbx3 gene are in exons 2 and 3. Row 2, conditional targeting vector. The neo-tk cassette is flanked by FRT sites (depicted as thin blue arrowheads) that can be used to excise the cassette by FlpE expression. The Cbx3 gene can be disrupted by Cre expression, which excises exons 2 and 3, containing the alternative ATG start sites. Row 3, Cbx3 genomic locus after targeting with the conditional vector shown in row 2, and represents the Cbx3hypoallele. Row 4, genomic locus after FlpE expression and removal of the neo-tk cassette. Row 5, genomic locus after Cre expression and excision of exons 2 and 3, giving rise to the Cbx3-/-null allele.