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:…ability of various urea transporters to handle different analogs of urea has proven to be diagnostic of the presence and categorization of urea transporters in fish (Schmidt-Nielsen and Rabinowitz 1964; Wood et al. 1998; McDonald et al. 2000, 2002, 2003; Pilley and Wright 2000; Walsh et al. 2001a).
Category: