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:A 3-day duration of UUO was used, as both AT1 and AT2 receptors are abundant in the kidney at this age and renal AT2 expression falls rapidly in the neonatal period (27, 48).
Category: