We have analyzed reticulocyte and leukocyte mRNAs isolated from a patient with congenital methemoglobinemia and pseudohermaphrodism. The cytochrome b5 cDNA sequences were amplified using specific oligonucleotide primers and the polymerase chain reaction (PCR). DNA sequencing indicated that there was a 16-bp deletion in the cDNA leading to a new, in-frame stop signal and resulting in a truncated protein of 45 amino acids. Genomic DNA was analyzed, and the molecular lesion was shown to be an AG-->GG alteration in the 3' splicing junction of intron 1. The splice site alteration leads to the usage of the nearest AG as an alternative splice site, resulting in a 16-bp deletion in the mRNA. All of the studies on reticulocyte mRNA and genomic DNA indicated that the patient was homozygous for the lesion.