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:A series of nonfluorescing exo mutants that fail to produce succinoglycan were isolated; most of the exo genes are located in a cluster, which has been sequenced (10, 11, 12, 67, 68, 94, 103, 113).
Category: