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:We used the HB cell lines HUH6 (Japanese Collection of Research Bioresources, JCRB, Osaka, Japan), HepT1 [41], HepT3 [7], and HepG2 [42], as well as the hepatocellular carcinoma (HCC) cell line HUH7 (JCRB).
Category: