Fig. 1. Targeted disruption of the Six4 gene. (A) Schematic representation of the wild-type Six4-Six1 locus, with the restriction sites used in the present study. The 4.2 kb 5' and 3.1 kb 3' homologous fragments are indicated by double headed arrows. The Six4-loxP targeting fragment is shown, and the two potential targeted Six4 mutant alleles after Cre excision are depicted below. Schematic representations are not to scale. (B) Southern blot analysis of genomic DNA digested by PstI and hybridized with a 5' external probe (above) and a 3' external probe (below), before Cre recombination leading to a Six4^hygrogfp allele, or after Cre recombination leading to a Six4^gfp allele. (C) Phenotype of wild type (left) and Six1^–/–Six4^–/– (right) newborn mice.