At least 28 proteins have now been defined as collagens (Trends Genet. 20:33–43, 2004; J. Biol. Chem. 281:3494–3504, 2006), but many of those recently discovered are present in tissues in such small amounts that their isolation for characterization at the protein level has so far been impossible. Some of the fibrilforming collagens are used as a biomaterial in numerous medical applications and as a delivery system for various drugs (3, 4). The collagens used in all these applications have been isolated from animal tissues and are liable to cause allergic reactions in some subjects and carry a risk of disease-causing contaminants (3,4). An efficient recombinant expression system for collagens can thus be expected to have numerous scientific and medical applications. The systems commonly used for expressing other proteins in lower organisms are not suitable as such for the production of recombinant collagens, however, as bacteria and yeast have no prolyl 4-hydroxylase activity and insect cells have insufficient levels of it. Prolyl 4-hydroxylase, an α 2 β 2 tetramer in vertebrates, plays a central role in the synthesis of all collagens, as 4-hydroxyproline-deficient collagen polypeptide chains cannot form triple helices that are stable at 37°C (5,6). All attempts to assemble an active prolyl 4-hydroxylase tetramer from its subunits in vitro have been unsuccessful, but active recombinant human prolyl 4-hydroxylase has been produced in insect cells, yeast, and Escherichia coli by coexpression of its α - and β -subunits (7–9).