Essential component of glycosylphosphatidylinositol-mannosyltransferase 1 which transfers the first of the 4 mannoses in the GPI-anchor precursors during GPI-anchor biosynthesis. Probably acts by stabilizing the mannosyltransferase PIGM (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Interacts with PIGM. N-glycosylated. Belongs to the PIGX family. Truncated N-terminus. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon. Truncated N-terminus. Contaminating sequence. Potential poly-A sequence. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon.