Vigilin, a K homology (KH) protein has been found in all eukaryotic species studied. It has a unique structure of 14-15 consecutively arranged KH domains which apparently mediate RNA-protein binding. Cloning and sequencing of the mouse vigilin cDNA confirmed that the amino acid sequences of vertebrate vigilins are highly conserved and contain conserved sequence motifs of nuclear import and export sequences. The human and murine vigilin mRNAs carry two alternatively spliced 5' exons. In the 5' leader region of one of the splice variants, variant 1A, we found an upstream open reading frame (uORF) highly conserved between mouse and human. Here we present for the first time evidence that a 13 amino acid long peptide encoded by this uORF is an inhibitor of vigilin expression operating on a posttranscriptional level. We propose that the two structurally different 5' leader sequences of the human vigilin mRNA are involved in the regulation of vigilin biosynthesis.