‘Secondary’ hyperammonaemia may be a feature in other metabolic disorders like organic acidurias (e.g. methylmalonic aciduria, propionic aciduria) (Ribas et al. 2022) or carbonic anhydrase 5A-deficiency (CA5A gene) (van Karnebeek et al. 2014; Diez-Fernandez et al. 2016). The gene discussed is CA5A; the disease is hyperinsulinemic hypoglycemia, familial, 4.