The yeast TREX complex physically couples elongating RNA polymerase II with RNA processing and nuclear RNA export factors to facilitate regulated gene expression. Hpr1p is an essential component of TREX, and loss of Hpr1p compromises transcriptional elongation, RNA export, and genome stability. Despite these defects, HPR1 is not essential for viability in yeast. A functional orthologue of Hpr1p has been identified in metazoan species and is variously known as Thoc1, Hpr1, or p84. However, the physiological functions of this protein have not been determined. Here, we describe the generation and phenotypic characterization of mice containing a null allele of the Thoc1 gene. Heterozygous null Thoc1 mice are born at the expected Mendelian frequency with no phenotype distinguishable from the wild type. In contrast, homozygous null mice are not recovered, indicating that Thoc1 is required for embryonic development. Embryonic development is arrested around the time of implantation, as blastocysts exhibit hatching and blastocyst outgrowth defects upon in vitro culture. Cells of the inner cell mass are particularly dependent on Thoc1, as these cells rapidly lose viability coincident with Thoc1 protein loss. While Hpr1p is not essential for the viability of unicellular yeasts, the orthologous Thoc1 protein is required for viability of the early mouse embryo
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