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:Schuler et al. (1999, 2003) used two types of Brassica napus (oilseed rape/canola), a conventional variety and a transgenic variety that expressed Bt toxin Cry1Ac.
Category: