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:P346 construct [18] and subcloned into XhoI-ApaI restricted pIgHSV40s vector which contained the SV40 promoter and the immunoglobulin m heavy chain enhancer (Em).
Category: