(A) Generation of Er81EWS-Pea3 mutant mice. Above is the organization of the Er81 genomic locus in the region targeted by homologous recombination in analogy to [14]. Exons 1–4 are shown as light blue boxes, and the Er81 start codon in exon 2 is indicated as ATG. The probe used to detect homologous recombination is shown as a grey box. Below is replacement of Er81 by EWS-Pea3 through the integration of EWS-Pea3 in frame with the endogenous start codon of the Er81 locus in exon 2 (in analogy to [14]).