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 core components of the cell cycle machinery and the vital regulators driving cell cycle progression (Malumbres and Barbacid, 2001; Olashaw and Pledger, 2002; Chulu and Liu, 2009; Malumbres and Barbacid, 2009).
Category: