Baf57 allelic series produced by a single gene-trap vector. The domain structure of BAF57 is depicted at the top, where the N-terminal 23-residue proline-rich region (P) is encoded by Exons 2 to 4. BAF57 translation normally starts at the ATG in exon 2, but exon 4 (and exon 5) also contains an ATG that is in frame with the remaining sequence. The targeting vector contains a pair of Flippase recognition target (FRT) sites (red dots) and a pair of LoxP sites (triangles) flanking the gene-trap cassette. During homologous recombination, crossing over at the left arm can happen downstream or upstream of the 5' Lox P site (red vertical lines 1 and 2 flanking the 5' Lox P site), generating Baf57ΔR and the intermediate Baf57Int alleles, respectively, the latter convertible to Baf57F, where exons 2 and 3 are floxed. DT, diphtheria toxin expression cassette; HMG, high mobility group; NHRLI, a domain rich in Asp, His, Leu and Ile; NLS, nuclear localization signal. Green arrows denote PCR primers used in Figure 3A.