The bicistronic MOCS1 gene encodes two enzymatic activities that are necessary for the biosynthesis of the molybdenum cofactor (MoCo). Mutations in either of the two consecutive open reading frames are responsible for the majority of MoCo deficiency cases and result in a complementation group A phenotype. Two cDNA sequences have been described, which differ in the 5' sequence and encode for two forms of the protein MOCS1A with variable N-terminal sequences. We have reinvestigated the corresponding region by means of cDNA analysis and databank searches. This revealed three different splice variants, including two mutually exclusive first exons and a facultative intron. All three forms can be found in eight different human tissues in a constant ratio, which excludes tissue specificity of the different isoforms.