The mutation in this allele comprises a small translocation insertion that placed a 3.3 kb fragment of Chr 10 into intron 1 of the Tbx10 gene. This inserted sequence includes exon 1 of the Tebp gene. RT-PCR analysis of Tbx10 expression revealed that the full-length trancript is expressed normally in mutant mice, and that a chimeric transcript consisting of the Tebp exon 1 of the inserted sequence fused in-frame to exon 2 of the Tbx10 gene is expressed ectopically. This chimeric transcript is predicted to encode a trucated, but functional protein that is similar to a normal variant.