(B) Southern blot analysis of EcoRI-digested DNA from either wild-type ES cells (E14) or targeted ES cell clones bearing the Atrx flox allele (1/F12 and 1/G11) hybridised with either the 20/27 (left blot) or Hae0.9 (right blot) probes. The EcoRI fragment of the Atrx WT allele (18.5 kb) has been replaced with the expected fragments of 11.2 kb (20/27 probe) or 8.5 kb (Hae0.9 probe)