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:Along with proteins of the Bcl-2 family, other mitochondrial proteins are known to play a key role in mammalian cell apoptosis (Kroemer and Reed, 2000; Wang and Zhang, 2001, reviewed in Bayir and Kagan, 2008; Ola et al., 2011).
Category: