In a span of approximately 13 kb, two genomic fragments showing an obvious overlap encoded the 1,192 base pair (bp) cDNA sequence for human H-protein. Because of the close similarity in the genomic organization comprised of five exons to that for the chicken H-protein gene, this region was assigned to the true H-protein gene in human. Primer extension analysis suggested and S1 protection analysis confirmed that at least one additional exon for the 74 bases-long 5' untranslated region in H-protein mRNA exists as the presumable first exon. None of the 5.0- and 5.5-kb SacI and the 5.2-kb EcoRI fragments which were undetectable in the genomes of patients with nonketotic hyperglycinemia was included in the true H-protein gene.