Glutathione synthetase (GSS) deficiency is an extremely rare autosomal recessive inborn error of metabolism. This metabolic disorder is caused by mutations in the GSS gene, which encodes GSS—an enzyme that catalyses the ATP-dependent conversion of γ-glutamyl-cysteine and glycine to glutathione. Low glutathione impairs the cellular antioxidant defence mechanism, leaving cells susceptible to oxidative stress and damage.