Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Although the distribution of the two collagen types is widely overlapping, important differences have been described, including a high ratio of type VI to type I collagen in intervertebral disks and articular cartilage, and an opposite proportion in bone and a preferred localization of type VI collagen in the pericellular space or underneath epithelial cell layers (Keene et al., 1988; Marvulli et al., 1996).
Category: