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:However, by 3 days of UUO, renal AT1 and AT2 receptor expression is not different from that of the intact kidney, whereas by 28 days of UUO, AT1 receptor mRNA and binding have increased compared with the intact kidney (48).
Category: