As a visible marker, early linkage studies with the original droopy ear allele or the deH allele identified a map position in the middle of Chromosome 3, distal to matted and proximal to Varitint-waddler (Carter and Falconer 1951; Curry 1959; Lane and Eicher 1979; Holmes et al. 1981). We used an F2 intercross with CAST/Ei mice to localize deH to a 0.1 cM interval between D3Mit213 and 16.MMHAP32FLF1, which was refined by development of a bacterial artificial chromosome (BAC) contig and additional markers to a 1.4 Mb region that contained eight genes, including Tbx15 (Figure 3A). We considered Tbx15 as an excellent candidate for the skeletal abnormalities caused by deH, based on studies by Agulnik et al. (1998), who described its embryonic expression in the craniofacial region and developing limbs.