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, it is unlikely that CEP290 and BBS4 play a redundant role on PCM-1 function because knockdown phenotypes are distinct: the depletion of CEP290 resulted in an abnormal concentric accumulation of PCM-1, whereas BBS4 knockdown caused the dispersion of PCM-1 (20).
Category: