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:The Western immunoblotting procedure was used to localize and quantify Ca2 -ATPase, PLN, CaM kinase II, and calsequestrin in SR membrane vesicles and cardiac muscle homogenates according to the procedure described by Xu and Narayanan (59).
Category: