Alpha 1,3/1,6 mannosyltransferase ALG2 (ALG2) is a bifunctional mannosyltransferase which normally transfers a mannose moiety to the lipid linked oligosaccharide (LLO aka N glycan precursor) which is required for subsequent N glycosylation of proteins. Defects in ALG2 can cause congenital disorder of glycosylation 1i (ALG2-CDG, previously known as CDG1i; MIM:607906), a multisystem disorder characterised by under glycosylated serum glycoproteins. Two mutations causing ALG2-CDG have been identified in a patient; a compound heterozygote where one mutation is a 1-bp deletion (G) at 1040 (p.G347Vfs*27) and the other a G-T transversion at 393 (not shown) (Thiel et al. 2003). Defect in ALG2 can also cause congenital myasthenic syndrome (ALG2-CMS), which is due to a defect in neuromuscular signal transmission (Cossins et al., 2013). The most commonly affected muscles include proximal limb muscles. Mutations causing ALG2-CMS include p.V68G and p.72_75delinsSPR (Cossins et al., 2013).