In animals, but also other eukaryotes, both enzyme activities are catalyzed by a single mitochondrial bifunctional enzyme named α-aminoadipic semialdehyde synthase [1], which is encoded by the gene AASS. One mutation in AASS has been reported to cause hyperlysinemia (c.1601_1609del; p.C534X [2]). Here, AASS is linked to hyperlysinemia.