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:CTL activity was measured in a standard 4 h 51Cr-release assay, as described previously (Kammer et al., 1999; Moradpour et al., 2001; Gremion et al., 2004).
Category: