Appropriate expression of the selenocysteine tRNA (tENASec) gene is necessary for the production of an entire family of selenoprotein enzymes. This study investigates the consequence of disrupting an upstream enhancer region of the mouse tRNASec gene (Trsp) known as the distal sequence element (DSE) by use of a conditional repair gene targeting strategy, in which a 3.2-kb insertion was introduced into the promoter of the gene. In the absence of DSE activity, homozygous mice failed to develop in utero beyond embryonic day 7.5 and had severely decreased levels of selenoprotein transcript. Cre-mediated removal of the selection cassette recovered DSE regulation of Trsp, restoring wild-type levels of tRNASec expression and allowing the generation of viable rescued mice. Further analysis of targeted heterozygous adult mice revealed that the enhancer activity of the DSE is tissue dependent since, in contrast to liver, heart does not require the DSE for normal expression of Trsp. Similarly, in mouse cell lines we showed that the DSE functions as a cell-line-specific inducible element of tRNASec. Together, our data demonstrate that the DSE is a tissue-dependent regulatory element of tRNA Sec expression and that its activity is vital for sufficient tRNASec production during mouse embryogenesis.