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:CDKs are enzymatically active when associated with cyclins, and three sets of cyclin–CDK complexes including cyclin D–CDK4, cyclin D/CDK6 and cyclin E–CDK2 complexes are sequentially assembled and activated during the G1 phase (Malumbres and Barbacid, 2001; Chulu and Liu, 2009).
Category: