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:1 was expressed with ABI1-HA or mER7 control DNA (Yoo et al., 2007) in 3 mL of protoplasts (6 3 105 cells) under standard conditions.
Category: