Direct cDNA sequencing has been performed on asymmetrically amplified transcripts from the human porphobilinogen deaminase gene. Lymphocytes from 30 patients with acute intermittent porphyria were the source of mRNA; of the seven separate point mutations detected, three were silent, whereas four resulted in amino acid changes. Three of these changes involved highly conserved amino acids, and the remaining one a conserved charge. One of these mutations was predicted to cause structural alterations in the protein product. The application of this method to affected families allows the direct identification of these heterogeneous mutations, thus permitting the unequivocal detection of carriers.