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 contrast, retinoic acid up-regulates keratin 13 and 19, which are normally found in non-keratinized epithelia (Kopan et al. 1987, Asselineau et al. 1989).
Category: