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:The mechanism of action probably relates, at least in part, to up regulation of the cyclin kinase inhibitor p21WAF1/CIP1 in HDAC1 null cells (Lagger et al., 2002).
Category: