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, CVP2 has higher catalytic activity than CVL1 in vitro and the cvp2 mutant shows early termination of vascular elongation, while the cvl1 mutant resembles wild type, suggesting an unequal redundancy in which CVP2 plays a more significant role in PtdIns(4,5)P2 degradation and PtdIns(4)P production (Carland and Nelson, 2009; Carland et al., 1999; Carland and Nelson, 2004).
Category: