(D) Diagram of Tbx15LacZ allele constructed by gene targeting. As described in the text, this allele is predicted to give rise to a protein truncated after approximately 154 codons and is lacking critical residues of the T box. Heterozygotes for the targeted allele exhibit normal size, morphology, and hair-color patterns, but homozygotes and Tbx15LacZ/deH compound heterozygotes are identical to deH homozygotes.