Alkaptonuria (AKU) is an ultra‐rare metabolic disease in which a deficiency of the enzyme homogentisate 1,2‐dioxygenase leads to the accumulation of homogentisic acid (HGA) (La Du, Zannoni, Laster, & Seegmiller, 1958); for review see Gallagher, Dillon, Sireau, Timmis, and Ranganath (2016). Here, HGD is linked to alkaptonuria.