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:Mice deficient in cd39 on the C57BL/ 6/129 svj strain or in cd73 on the BL6 stain were generated, validated, and characterized as described previously (15, 16).
Category: