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:Furthermore, it has been shown that a minimum of 200–300 amino acids from the C-terminal of HrpZ from P. syringae pv. phaseolicola (HrpZPph) are required for the induction of HR (Lee et al. 2001a) implying that several functional domains exist in HrpZPph (Lee et al. 2001b).
Category: