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:In mammalian cells, cyclin D–CDK4/6 and cyclin E–CDK2 complexes are required to promote cell cycle entrance from quiescence, progression through the G1 phase and transition from G1 into S phase in response tomitogenic stimulation (Sherr, 1995; Malumbres and Barbacid, 2001; Chulu and Liu, 2009).
Category: