Fig. 1. Homeobox swap strategy. (A) Organization of the genes of the HoxA cluster. (B) Amino acid sequence comparison of the A11 and A13 homeodomains. Dashes represent sequence identity. (C) The targeting construct at the top consisted of the A11 gene with a substituted A13 homeodomain (HD^*, black rectangle), inserted loxP flanked Neo and HSV thymidine kinase gene. Double homologous recombination, as shown, results in homeobox swap and insertion of loxP flanked Neo. Mating with transgenic Cre mice removes Neo, leaving a single 34 bp loxP. PCR genotyping used primers a-d, and Southern blot genotyping used restriction sites (P) PstI, (N) NheI and DNA segment probes (open lines and solid rectangles).