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:MMP-2 is considered a constitutively expressed MMP, whereas MMP-9 is expressed at low levels under normal conditions (Gottschall et al., 1995; Yong et al., 1998; Planas et al., 2001) but is induced under pathological conditions, such as cerebral ischemia and neurodegeneration (Aoki et al.
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