HMBS and variegate porphyria: Acute hepatic porphyria (AHP) is a group of four distinct metabolic diseases caused by genetic defects in the heme synthesis pathway (1): The most common subtype, Acute Intermittent Porphyria (AIP), caused by a mutation in the HMBS gene, followed by Variegate Porphyria (VP), resulting from a PPOX gene mutation and Hereditary Coproporphyria (HCP), caused by a mutation in the CPOX gene.