Low cholesterol is a common feature among patients with congenital disorders of glycosylation (CDG), but the exact underlying mechanisms remain unknown. We explored the role of proprotein convertase subtilisin/kexin type 9 precursor (PCSK9), a major cholesterol regulator, in mediating low cholesterol among CDG patients. First, we describe a CDG patient with low circulating PCSK9 levels. Second, we verify this observation in 8 additional CDG patients. Third, we conduct experimental studies showing that loss of PCSK9 N-glycosylation reduces extracellular PCSK9 activity. Altogether, epidemiological and experimental findings suggest that PCSK9 may partially mediate low cholesterol among CDG patients. (By Michael Chong, https://jmg.bmj.com/content/early/2019/08/06/jmedgenet-2019-106102 )