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:These cells, however, were subsequently identified as pigmented epithelial cells, which could form clonogenic spheres but failed to differentiate into retinal neurons (Cicero et al., 2009; Gualdoni et al., 2010).
Category: