Because of its unusual tensility, collagen has made its way into products as diverse as desserts, shampoo, and bioengineered skin. Usually collagen is extracted from cow hooves and hide, but for medical applications, sources other than rendering lots are needed to ensure the supply is highly purified, active, and pathogen-free. Bulleid and coworkers have turned to mouse mammary glands to produce a recombinant form of procollagen in mouse milk (see p. 385). By co-expressing a collagen-hydroxylating enzyme, they produced the mature thermostable form of the protein.