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:…of non-genic SNPs. Gene classes were defined according to PharmaADME core list and related gene list (pharmaADME.org), CYP genes metabolizing primarily exogenous or endogenous substrates were defined based on information from the literature (Nebert and Russell 2002; Guengerich 2003; Thomas 2007).
Category: