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:Many Sulfolobus chromatin proteins and transcription factors are post-translationally modified (Bell et al., 2002; Botting et al., 2010; Esser et al., 2012; Reimann et al., 2012; Niu et al., 2013; Reimann et al., 2013).
Category: