Using sequence information from Agulnik et al. (1998) and the partially completed mouse genome sequence, we found that portions of several Tbx15 exons could not be amplified from deH/deH genomic DNA. The same gene was initially referred to as Tbx8 (Wattler et al. 1998) and then later renamed Tbx14, but is currently referred to in several vertebrate genomes as Tbx15 (Agulnik et al. 1998; Begemann et al. 2002). By comparing the sequence of a 1.3 kb junction fragment amplified from deH/deH genomic DNA to publicly available mouse genome sequence, we identified a 216 kb deletion that extends from Tbx15 intron 1 to 148 kb downstream of the polyadenylation sequence in a region annotated as a mannose-6-phosphate receptor pseudogene, M6pr-ps (Figure 3B and 3C). (Ludwig et al. 1992). By Northern blot analysis, we identified a fusion transcript produced from the deH chromosome (data not shown). However, the deletion removes 534 of the 602 amino acids encoded by Tbx15 (including the T-box DNA-binding domain), deH/+ animals are grossly normal, and the phenotype of deH/deH animals is identical to that described for the original allele. In addition, other than M6pr-ps, no other genes or transcripts have been annotated to the 216 kb deletion.