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:However, the broad substrate specificity of the AtPLT5 and the unchanged phenotype of Atplt5-knock-out plants make it difficult to speculate on the physiological role of AtPLT5 (Klepek et al. 2005; Reinders et al. 2005).
Category: