Collagen is the most abundant mammalian protein and the main component of connective tissues. Typically collagen is first synthesized as individual glycine-rich preprocollagen monomeric strands from the ribosomes in the rough endoplasmic reticulum (RER). After transport to the lumen of the RER signal peptides are cleaved to yield procollagen. Lysine and proline are hydroxylated and glycosylated in the lumen and a helix composed of three peptides is formed. It is then transported to the Golgi and excreted from the cell by exocytosis. Procollagen's propetides are then removed to yield tropocollagen which may join together to form fibrils. The "collagen type" designations of Bornstein and Traub is determined by the peptide composition of the triple helix.