We previously reported a patient with hereditary hypotransferrinemia who suffered from severe anemia and growth retardation and was diagnosed on the basis of an extremely low level of serum transferrin (TF). By an isoelectric focusing analysis, we found that the patient and his father shared a variant TF protein with an abnormal isoelectric point. The study suggested that the patient was a compound heterozygote with a variant allele, encoding the mutant TF, of paternal origin and a null allele of maternal origin. In the present study, we investigated the TF gene of the patient and his family. We showed that the patient and his father shared a variant TF gene bearing a GAA to AAA transition at codon 394. This nucleotide substitution causes a nonconservative amino acid change from glutamate to lysine in amino acid residue 375 of the TF protein. This single amino acid mutation is predicted to cause a conformational change in the coiled region of the carboxyl-terminal iron-binding lobe. As for the maternal null allele, no mutation was found in either the coding region or the exon-intron boundaries, suggesting an abnormality in the transcription or stability of mRNA of maternal allele origin.