The human cystathionine beta-synthase (CBS) gene spans in excess of 30 kb and consists of 19 exons, with three different 5' untranslated regions including three different exons 1 (exons 1 a, b, and c). Exon 1a and 1b are 390 bp apart from each other and are linked to exon 2 in cDNA << a >> and cDNA << b >>. Exon 1c, which linked to exon 5 in cDNA << c >>, is 7 kb apart from exon 1b. All splice sites conform to the GT/AG rule, including those from exon 1a or 1b to exon 2 and from exon 1c to exon 5. Upstream of exons 1a and 1b, we found two putative promoter sequences with high C + G nucleotide content, one CAAT box at -70 nucleotides (for exon 1b), no TATA box, several Spl binding regulatory consensus sequences, and some other regulatory sequences. Human adult and fetal Northern blots hybridized with total cDNA containing exon 1b, or specific probes from exons 1 (b and c) showed mRNAs of 2.5 kb, 2.7 kb, and 3.7 kb. These results suggest that the mRNAs containing the different exons 1 are under the control of different promoters.