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:Serum levels of VEGF, PlGF, Flt-1, Flk-1, Ang-2 and Tie-2 were quantified by ELISA (Quantikine Human, R&D SYSTEMS, UK).
Category: