clk-1 has been identified and characterized in the nematode Caenorhabditis elegans as a gene that affects the rates, regularity, and synchrony of physiological processes. The CLK-1 protein is mitochondrial and is required for ubiquinone biosynthesis in yeast and in worms, but its biochemical function remains unclear. We have studied the expression of murine mclk1 in a variety of tissues, and we find that the pattern of mclk1 mRNA accumulation closely resembles that of mitochondrial genes involved in oxidative phosphorylation. The pattern of protein accumulation, however, is sharply distinct in some tissues; mCLK1 appears relatively enriched in the gut and depleted in the nervous tissue. We also show that mCLK1 is synthesized as a preprotein that is imported into the mitochondrial matrix, where a leader sequence is cleaved off and the protein becomes loosely associated with the inner membrane. However, in contrast to all known mitochondrial proteins that contain a cleavable pre-sequence, the import of mCLK1 does not require a mitochondrial membrane potential.