The molecular basis of the uroporphyrinogen III synthase (UROIIIS) deficiency was investigated in two members of a Japanese family. This defect in heme biosynthesis is responsible for a rare autosomal recessive disease: congenital erythropoietic porphyria (CEP) or Gnther's disease. The first patient was homoallelic for a novel missense mutation: a T to C transition of nucleotide 634 that predicted a serine to proline substitution at residue 212 (S212P). The second patient appeared heteroallelic, carrying the same missense mutation and a nonsense mutation: a C to T change at nucleotide 745, resulting in a premature stop at codon 249, instead of a glutamine (Q249X). The corresponding mutated proteins were expressed in Escherichia coli and no residual activity was observed. A family study was also performed to determine the carrier status.