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:This S1P gradient that regulates immune cell distribution (Zhao et al. 2015) and contributes to endothelial function (Rosenberg & Stern 1966) is maintained by conversion of S1P to sphingosine within cells by S1P phosphatases (Spp1 and Spp2) and extracellularly by lipid phosphate phosphatases (LPP1…
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