Mucopolysaccharidoses (MPS) are a group of genetic disorders where cells show progressive storage of protein-sugar-containing molecules. So far, nine different types of MPS have been described with various involvement of organs and skeleton, depending on the underlying genetic defect. We have identified a novel MPS subtype, i.e., subtype X, caused by pathogenic variants in the Arylsulfatase K (ARSK) gene in four patients with short stature, coarse facial features, cardiac and ophthalmologic anomalies, and normal intellectual abilities. ARSK-deficiency should be considered in patients with suspected MPS where no precise diagnosis could be established by testing for the hitherto known subtypes. (By Dr. Barbara Plecko, https://jmg.bmj.com/content/early/2021/12/15/jmedgenet-2021-108061 )