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:Skeletal muscle is also a primary site of disease in mouse models of ALS (Dobrowolny et al., 2008; Wong and Martin, 2010; Luo et al., 2013) and possibly in human ALS (Vielhaber et al., 2000).
Category: