The protein encoded by this gene is the fourth enzyme in the heme biosynthesis pathway. It converts hydroxymethylbilane to uroporphyrinogen III, a cyclic tetrapyrrole. This enzyme is defective in the autosomal recessive disorder congenital erythropoietic porphyria. Alternate promoter usage controls cell type-specific expression, including erythroid cell-specific expression. Alternative splicing results in multiple transcript variants encoding different isoforms. [provided by RefSeq, Sep 2014]