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:-Actin, renin, COX-1, COX-2, nNOS, and endothelial NOS (eNOS) mRNA levels were measured by ribonuclease protection assays, as described elsewhere (4, 26).
Category: