The collagens are a large and diverse family of proteins which are found in the extracellular matrix. In common with one another, the 19 known collagen types have triple-helical domains of variable length but they differ with respect to their overall size and the nature and location of their globular domains. Collagen mutations lead to heritable defects of connective tissues and mutation data for collagen types I and III are presented here. The mutation data are accessible on the world wide web at http://www.le.ac.uk/genetics/collagen/