The functionality of this variant was subject to numerous studies (e.g. Befort et al. 2001; Bond et al. 1998; Deb et al. 2010; Filbey et al. 2008; Margas et al. 2007; Ray et al. 2011). The 118G variant was associated with lower cell-surface receptor binding site availability (Beyer et al. 2004; Kroslak et al. 2007), reduced mRNA levels in vivo and in vitro (Zhang et al. 2005), altered signal transduction (PKA and pERK), in vitro (Deb et al. 2010), and enhanced affinity for beta-endorphin (Bond et al. 1998). For more information and references, see a current review (Mague and Blendy 2010).
