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:4 is able to regulate neurotrophic factors expression, namely brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) [10, 26-29].
Category: