T box (Tbx) genes are a family of developmental regulators with more than 20 members recently identified in invertebrates and vertebrates. Mutations in Tbx genes have been found to cause several human diseases. Our understanding of functional mechanisms of Tbx products has come mainly from the prototypical T/Brachyury, which is a transcription activator. We previously discovered ET, a Tbx gene expressed in Xenopus embryos. We report here that ET is an ortholog of the human Tbx3 and that ET is a repressor of basal and activated transcription. Functional dissection of the ET protein reveals a novel transcription-repression domain highly conserved among ET, human TBX3, and TBX2. These results reveal a new transcription repressor domain, show the existence of a subfamily of transcription repressors in the Tbx superfamily, and provide a basis for understanding etiology of diseases caused by Tbx3 mutations.