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:Transformation by GSTO1-1 is the rate-limiting step in inorganic-arsenic metabolism and requires glutathione as a reductant (Cohen et al., 2006; Pott et al., 2001; Tapio and Grosche, 2006; Tseng, 2007).
Category: