Phosphatidylinositolglycan class N (PIG-N) is a mammalian homolog of the yeast protein MCD4P and is expressed in the endoplasmic reticulum. PIG-N is essential for glycosylphosphatidylinositol anchor synthesis. Glycosylphosphatidylinositol (GPI)-anchored proteins are cell surface-localized proteins that serve many important cellular functions.