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 concentration of katanin in mitotic spindle poles (McNally and Thomas, 1998) and the specific activation of katanin’s activity in Mphase Xenopus extracts (Vale, 1991; McNally and Thomas, 1998) suggest that katanin may also sever microtubules from their centrosomal attachments during mitosis in non-neuronal cells.
Category: