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:When the ROS content is excessively high in plant cells, NO by itself can interact with O2 - to become peroxynitrite (ONOO-) (Lipton et al. 1993; Yamasaki et al. 1999).
Category: