Catalyzes the addition of the first of nine mannose moieties to form a dolichol-lipid linked oligosaccharide intermediate required for proper N-linked glycosylation. GDP-alpha-D-mannose + N,N'-diacetylchitobiosyl diphosphodolichol = beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-alpha-D-GlcNAc-diphosphodolichol + GDP + H(+) Protein modification; protein glycosylation. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 33 subfamily.