POMT1 (Protein O mannosyl transferase 1) is a multipass membrane protein that is found in the endoplasmic reticulum. POMT1 catalyses the transfer of mannosyl residues to the hydroxyl groups of serine or threonine residues. Enzymatic activity is dependent on co expression of POMT1 with POMT2. Defects in the POMT1 gene are associated with Walker-Warburg syndrome (WWS), a congential muscular dystrophy that is associated with mental retardation and is usually lethal within the first few months of life. Other defects in the POMT1 gene result in limb girdle muscular dystrophy type 2K (LGMD2K), which is associated with mild mental retardation. Studies in Drosophila suggest that mutation of POMT1 alters the efficacy of synaptic transmission and a change in subunit composition of post synaptic glutamate receptors at the neuromuscular junction. Missense mutations in POMT1 have been associated with glioneuronal and glial brain tumours.