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:cells expressed CD7 in coculture with OP9 stroma cells in the absence of IL-7 and in the presence of IL-15 conditioning [(2) NL/(2)IL-7(+)IL-15].
Category: