Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the first alpha-1,4-mannose to GlcN-acyl-PI during GPI precursor assembly. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. The disease is caused by variants affecting the gene represented in this entry. Belongs to the PIGM family.