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:…intriguing are the recent observations showing
extensive polymorphisms in the circadian clock genes in humans including Period (Per) genes that have been shown to be involved in alcohol-induced changes in circadian rhythms in rodents (Perreau-Lenz et al., 2009; Spanagel et al., 2005a,b).
Category: