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:Lee et al. (2001) Human lung squamous cell carcinoma (CH27), human lung nonsmall cell carcinoma (H460) CH27 and H460 cells were treated with aloeemodin or emodin.
Category: