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:The large majority of T6SS genes (12 out of 17), including VipA, VipB, ClpV, VasF and VasK, are required for Hcp secretion, killing of amoeba and bacteria and are predicted to encode structural T6SS components [9,12,20].
Category: