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:To date, a handful of factors derived from testicular somatic cells, such as Sertoli cells, peritubular myoid cells, and Leydig cells, have been demonstrated to influence SSC renewal, in particular glial cell-line derived neurotrophic factor (GDNF), fibroblast growth factor 2 (FGF2), colony stimulating factor 1, and Wnt5a [4,5,6,7].
Category: