Congenital erythropoietic porphyria (CEP) or Günther's disease is an inborn error of heme biosynthesis transmitted as an autosomal recessive trait and characterized by a profound deficiency of uroporphyrinogen III synthase (UROIIIS) activity. Six missense mutations in the UROIIIS gene, a deletion and an insertion have already been described in CEP. This work brings further evidence for the heterogeneity in the genetic defect found in CEP. Two new mutations are described, a point mutation (V99A) and a frame-shift mutation (633insA) in the same patient who had a mild to moderate form of Günther's disease. The mutation (V99A) had a detectable residual activity when expressed in Escherichia coli while the insertion (633insA), which introduced a premature stop, had no activity. In the patients studied in our laboratory, the mutation C73R, associated with a severe phenotype, remains the most frequently seen.