ASS1 and mucopolysaccharidosis type 3A: Based on a systematic data collection, currently more than 100 disease‐causing ASS1 genetic variants are known; however, the impact of the genotype on the phenotypic presentation remains insufficiently understood.11 Given that residual enzymatic activities have already been reported to predict disease severity and survival rates in other inborn errors of metabolism, such as Farber disease and mucopolysaccharidosis type IIIA and VII,12, 13, 14 we hypothesized that ASS1 enzyme activity may correlate with disease severity in CTLN1.