C ongenital erythropoietic porphyria, an autosomal recessive inborn error of heme biosynthesis, results from the markedly deficient activity of uroporphyrinogen III synthase. Extensive mutation analyses of 40 unrelated patients only identified approximately 90% of mutant alleles. Sequencing the recently discovered erythroid-specific promoter in six patients with a single undefined allele identified four novel mutations clustered in a 20-bp region: (a) a –70T to C transition in a putative GATA-1 consensus binding element, (b) a –76G to A transition, (c) a –86C to A transversion in three unrelated patients, and (d) a –90C to A transversion in a putative CP2 binding motif. Also, a –224T to C polymorphism was present in approximately 4% of 200 unrelated Caucasian alleles. We inserted these mutant sequences into luciferase reporter constructs. When transfected into K562 erythroid cells, these constructs yielded 3 ± 1, 54 ± 3, 43 ± 6, and 8 ± 1%, respectively, of the reporter activity conferred by the wild-type promoter. Electrophoretic mobility shift assays indicated that the –70C mutation altered GATA1 binding, whereas the adjacent –76A mutation did not. Similarly, the –90C mutation altered CP2 binding, whereas the –86A mutation did not. Thus, these four pathogenic erythroid promoter mutations impaired erythroid-specific transcription, caused CEP, and identified functionally important GATA1 and CP2 transcriptional binding elements for erythroid-specific heme biosynthesis.