A method of producing a desired procolagen or derivative thereof in a systemwhich co-expresses and assembles at least one further procollagen orderivative thereof. The gene(s) for expressing pro-.alpha. chains orderivatives thereof for assembly into the desired procollagen has or have beenexogenously selected from natural pro-.alpha. chains or exogenouslymanipulated such as to express said pro-.alpha. chains or derivatives thereofwith domains which have the activity of C-terminal propeptide domains butwhich will not co-assemble with the C-terminal propeptide of the pro-.alpha.chains or derivatives thereof that assemble to form the said at least onefurther procollagen or derivative thereof.
