Chitobiosyldiphosphodolichol beta-mannosyltransferase (ALG1) normally tranfers a mannose moiety to the lipid-linked oligosaccharide (LLO aka N-glycan precursor) which is required for subsequent N-glycosylation of proteins. Defects in ALG1 can cause congenital disorder of glycosylation 1k (ALG1-CDG, previously known as CDG1k; MIM:608540), a multisystem disorder characterised by under-glycosylated serum glycoproteins. CDG type 1 diseases result in a wide variety of clinical features, such as defects in the nervous system development, psychomotor retardation, dysmorphic features, hypotonia, coagulation disorders, and immunodeficiency. Compared to other CDGs, ALG1-CDG has a very severe phenotype, which can result in an early death. Mutations in ALG1 causing ALG1-CDG include S258L, G342P, S150R, M377V, G145D, C396* and R276W (Schwarz et al. 2004, Kranz et al. 2004, Grubenmann et al. 2004, Dupre et al. 2010).