Delta-aminolevulinate synthase 1 is an aminolevulinic acid synthase. Delta-aminolevulinate synthase catalyzes the condensation of glycine with succinyl-CoA to form delta-aminolevulinic acid. This nuclear-encoded mitochondrial enzyme is the first and rate-limiting enzyme in the mammalian heme biosynthetic pathway. There are 2 tissue-specific isozymes: a housekeeping enzyme encoded by the ALAS1 gene and an erythroid tissue-specific enzyme encoded by ALAS2.

ALAS1 encodes a housekeeping form of ALAS inasmuch as it is expressed in all cell types, including erythroid cells, whereas ALAS2 encodes a red cell-specific form. ALAS1 and ALAS2 have only 59% identity in amino acid sequence; there was no homology in the amino-terminal region and about 73% homology after hepatic residue 197.