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:Additionally, cvp2 single mutants contained a point mutation in the cvp2 gene that results in expression of a nonfunctional protein (Carland and Nelson, 2004), and cvl1-1cvp2 double mutants with the above mutations were obtained.
Category: