This allele is derived from Tbx5tm1Jse. The floxed exon 3 and neo cassette were removed by crossing mutant animals to the cre-expressing transgenic line TgN(EIIa-cre)C5379Lmgd. This results in deletion of the N-terminal region of the T-box encoded by exon 3. RT-PCR analysis of mutant animals detected a truncated transcript lacking exon 3.