O-linked mannose beta-1,4-N-acetylglucosaminyltransferase that transfers UDP-N-acetyl-D-glucosamine to the 4-position of the mannose to generate N-acetyl-D-glucosamine-beta-1,4-O-D-mannosylprotein (By similarity). Involved in the biosynthesis of the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity (PubMed:24256719). 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl-(1->4)-alpha-D-mannosyl)-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Mainly expressed in the central nervous system. Expressed during brain development. At 14.5 dpc, expressed throughout the cortical plate and the developing brain, as well as throughout the eye, including the lens and cornea. Cortical expression is already detected at 12.5 dpc, peaks between 14.5 and 16.5 dpc and is reduced at birth. Newborn are slightly smaller and die within the first day of birth due to abnormal basal lamina formation and neuronal migration defects. Defects are due to a lack of laminin-binding glycans. Belongs to the glycosyltransferase 61 family. Truncated C-terminus.