Synthesis of the GPI-precursors proceeds in a stepwise manner. Over 29 proteins have been shown to be involved in the biosynthetic pathway of GPI-anchor biosynthesis.¹⁵ It starts on the cytoplasmic side of the endoplasmic reticulum (ER). The addition of the GPI-anchor precursor to the carboxy-terminus of the protein from which a block of 17–31 amino acids has been cleaved occurs on the luminal side of the ER. Insert shows the structure of mammalian GPI-anchored proteins. GPI-anchors have a conserved structure (NH₂-CH₂-CH₂-PO₄-6Manα1–2Manα1–6Manα1–4GlcNα1–6myo-inositol-PO₄-lipid), which can be modified by the addition of an acyl chain to the inositol (not shown) and phosphoethanolamine to the first and possibly second mannose.