Mammalian TBP consists of a 180 amino acid core that is common to all eukaryotes, fused to a vertebrate-specific N-terminal domain. We generated mice having a modified tbp allele, tbp(DeltaN), that produces a version of TBP lacking 111 of the 135 vertebrate-specific amino acids. Most tbp(DeltaN/DeltaN) fetuses (>90%) died in midgestation from an apparent defect in the placenta. tbp(DeltaN/Delta...