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:…sulfoxide (Kitamura et al. 1977; Kimura et al. 1979), 2,5-dichlorophenylmethyl sulfone (Kitamura et al. 1977; Kimura et al. 1979) and mercapturic acid derivatives (Hawkins et al. 1980; Klos and Dekant 1994; Hissink et al. 1997) have also been identified as urinary metabolites.
Category: