HMBS and acute intermittent porphyria: HCP is clinically similar to other acute hepatic porphyrias: intermittent acute porphyria, variegate porphyria, and δ-aminolevulinic acid dehydratase porphyria, which are caused by mutations in the genes for porphobilinogen deaminase, protoporphyrinogen oxidase, and δ-aminolevulinic acid dehydratase, respectively.