Overall, 205 individuals with UCDs [male ornithine transcarbamylase deficiency (mOTC-D): n = 54, 26.4%; female OTC-D (fOTC-D): n = 39, 19.0%; argininosuccinate synthetase 1 deficiency (ASS1-D): n = 64, 31.2%; argininosuccinate lyase deficiency (ASL-D): n = 48, 23.4%] have a mean z-score within the normal range for birth weight, length and head circumference [mean z-score (birth weight): − 0.24; mean z-score (birth length): 0.14; mean z-score (birth head circumference): − 0.13] (Fig. 1A–C). Here, ASS1 is linked to hyperinsulinemic hypoglycemia, familial, 4.