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:Third, PWM-20 is based entirely on the functional p53 REs (34 in vivo REs used for PWM-20 compared to 37 in vitro BSs used by Hoh et al. [64].)
Category: