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:However, localization of additional
γ-tubulin that surrounds the centrosomes only during mitosis does require microtubules (McNally and Thomas, 1998; Khodjakov and Rieder, 1999).
Category: