To investigate the genomic organization around amino acid 96 where liver and erythrocyte cytochrome b5mRNA's are being spliced in a tissue specific manner, genomic clones containing this region were isolated and characterized. Two small exons and introns flanking these exons were identified and characterized. These exons were 29 and 36 nucleotides long. Splicing junctions between these exons showed unusual features comparing to known consensus sequences. The nucleotide sequence indicated that this could be the result of polymorphism present in a particular person because both CCA and CCG code for proline. The finding of unusual splicing junctions and polymorphism should facilitate the understanding of biosynthetic relationship between liver and erythrocyte cytochrome b5s and the design of accurate diagnostic methods for congenital methemoglobinemia at DNA level.