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:S1P also regulated ERK activity in several types of the cells including smooth muscle cells (Pyne et al., 1996) and osteoblasts (Kozawa et al., 1997) in addition to 3T3 fibroblasts (Wu et al., 1995; Blakesley et al., 1997; Rani et al., 1997).
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