Congenital methemoglobinemia caused by an erythrocytic deficiency of cytochrome b5 reductase (b5R; type I) in African-American individuals was first reported by this laboratory. The rarity of this observation is possibly due to the difficulty detecting cyanosis that is masked by naturally occurring dark skin pigment. Since previous biochemical studies on the African-American family with variant enzyme b5R-Shreveport showed enzyme instability, we focused on molecular analysis of its transcript. The transcript size was the same as that of a normal control. The nucleotide sequence of both normal and variant transcripts were examined by directly sequencing reverse transcriptase-polymerase chain reaction (RT-PCR) product. The propositus was found to be homozygous for a G to A transition at codon 212 in exon 8, changing a glutamate to a lysine (E212K). In addition, a C to G transversion was found at codon 116 in exon 5, changing a threonine to a serine (T116S). Using allele-specific PCR, we determined that E212K was found only in the propositus and her heterozygous mother. Furthermore, E212K is predicted to disrupt an alpha-helix peptide structure of b5R, suggesting that this is likely the disease-causing mutation. In contrast, T116S was found to be a high-frequency polymorphism specific for the African-American population. The E212K mutation is uniquely present in the 3' end of the b5R gene (exon 8), which differs from those b5R mutations found among Japanese subjects (exons 3 and 5) and in an Italian subject (exon 4) and, thus, further contributes to our understanding of the structure/function relationship of this housekeeping enzyme.