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:FGF23 Activates FGFR4 and PLCg Signaling in the Absence of a-Klotho To study FGF-FGFR-dependent signaling, we used HEK293 cells that express all FGFR isoforms but lack a-klotho (data not shown), similar to cardiac myocytes (Faul et al., 2011).
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