The function of PIGB (phosphatidylinositol glycan anchor biosynthesis class B, ENSG00000069943) is as follows. Alpha-1,2-mannosyltransferase that catalyzes the transfer of the third mannose, via an alpha-1,2 bond, from a dolichol-phosphate- mannose (Dol-P-Man) to a 2-acyl-6-[alpha-D-mannosyl-(1->6)-2- phosphoethanolamine-alpha-D-mannosyl-(1->4)-alpha-D-glucosaminyl]-1-(1- radyl,2-acyl-sn-glycero-3-phospho)-1D-myo-inositol intermediate to generate a 2-acyl-6-[alpha-D-mannosyl-(1->2)-alpha-D-mannosyl-(1->6)-2- phosphoethanolamine-alpha-D-mannosyl-(1->4)-alpha-D-glucosaminyl]-1-(1- radyl,2-acyl-sn-glycero-3-phospho)-1D-myo-inositol (also termed H6) and participates in the nineth step of the glycosylphosphatidylinositol- anchor biosynthesis (Probable). May also add the third mannose to a 2- acyl-6-[alpha-D-mannosyl-(1->6)-alpha-D-mannosyl-(1->4)-alpha-D- glucosaminyl]-1-(1-radyl,2-acyl-sn-glycero-3-phospho)-1D-myo-inositol (also termed H3) intermediate generating a 2-acyl-6-(alpha-D-mannosyl- (1->2)-alpha-D-mannosyl-(1->6)-alpha-D-mannosyl-(1->4)-alpha-D- glucosaminyl)-1-(1-radyl,2-acyl-sn-glycero-3-phospho)-1D-myo-inositol (also termed H4) (Probable). {ECO:0000305|PubMed:17311586, ECO:0000305|PubMed:8861954}.