In addition to reduced PBGD activity, symptomatic acute intermittent porphyria requires a marked induction of the housekeeping isoform of the first enzyme in the heme biosynthetic pathway, ALAS1, which is upstream of PBGD in the heme synthesis pathway and the rate-limiting enzyme for heme synthesis in the liver. Here, ALAS1 is linked to acute intermittent porphyria.