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:However, the relationships between those polymorphisms and susceptibility to AS remain unclear; some studies on association between PD-1, CTLA-4 polymorphisms and AS risk had used comparatively small samples [17–24], and the results about the role of PD-1 or CTLA-4 gene polymorphisms in AS patients are inconsistent and inconclusive.
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