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:In particular, mesenchymal fibroblast growth factor–10 (34), endodermally derived fibroblast growth factor receptor–2 (35), sonic hedgehog (SHH)/GLI 2,3 (36), and retinoic acid receptors (37) each play important roles during early tracheal–pulmonary morphogenesis.
Category: