Stabilizing subunit of the glycosylphosphatidylinositol- mannosyltransferase I complex which catalyzes the transfer of the first mannose, via an alpha-1,4 bond from a dolichol-phosphate-mannose (Dol- P-Man) to the glucosaminyl acyl phosphatidylinositol (GlcN-(acyl)PI) intermediate to generate alpha-D-Man-(1->4)-alpha-D-GlcN-(1->6)-(1- radyl,2-acyl-sn-glycero-3-phospho)-2-acyl-inositol and participates in the sixth step of the glycosylphosphatidylinositol-anchor biosynthesis. Probably acts by stabilizing the mannosyltransferase PIGM. . This is the function of PIGX (phosphatidylinositol glycan anchor biosynthesis class X, Ensembl gene identifier ENSG00000163964).