Congenital erythropoietic porphyria (CEP) is an autosomal recessive inborn error of metabolism that results from the markedly deficient activity of the fourth enzyme in the heme biosynthetic pathway, uroporphyrinogen III synthase (URO-synthase). To date, 17 mutations have been described including 11 missense, one nonsense, two mRNA splicing defects, one deletion and two coding region insertions. Most mutations have been identified in one or a few unrelated families with the exception of C73R and L4F which occurred in 29.6% and 9.3% of the 54 mutant alleles studied, respectively. Interestingly, analysis of the mutant alleles identified only 83% of the causative mutations, suggesting that about 20% of the mutations causing CEP lie elsewhere in the gene. Of note, mutation V82F, resulting from a G to T transversion of the last nucleotide of exon 4, caused both a missense mutation and an aberrantly spliced RNA transcript. Prokaryotic expression of the mutant URO-synthase alleles identified those with significant residual activity, thereby permitting genotype/phenotype predictions for this clinically heterogeneous disease.