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:The gross distribution of PLCb1-immunohistochemical staining in the rat central nervous system described here is consistent with previous evidences in mice, showing a predominant expression of PLCb1 protein (Gerfen et al., 1988; Hannan et al., 1998; Hozumi et al., 2008; Fukaya et al., 2008) in cerebral cortex, caudate–putamen and hippocampus, along with lower levels in diencephalon, brainstem and cerebellum.
Category: