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:Initial observations suggested this cell population possesses retinal stem cell properties, yet further analysis demonstrated that these cells could not differentiate into retinal neurons in vitro or in vivo (Cicero et al., 2009; Gualdoni et al., 2010).
Category: