Furthermore, mitochondrial catabolism of glutamine can rescue tumor cells from asparagine deficiency by providing the four carbons and two nitrogens required for asparagine synthesis.23 What’s more, glutamine is required for the resynthesis of asparagine from aspartate by asparagine synthetase (ASNS).24 ASNS expression is normally low in cells but activation of the GCN2-eIF2á system promotes translation of the transcription factor ATF4 (ref. 25) which induces ASNS expression. Here, ASNS is linked to hyperinsulinemic hypoglycemia, familial, 4.