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:…surface expression of NR1 subunit of NMDA receptor
EphB2 is known to control the surface expression of the NMDAR NR1 subunit in hippocampal neurons (Ciss e et al., 2011; Nolt et al., 2011), expression necessary for the induction of a robust form of long-term hippocampal potentiation (Klein, 2008).
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