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:Our results clearly demonstrated that PjPORB gene expression was subjected to leaf developmental control; and they were basically consistent with previous observations that PORB was strongly expressed early in seedling development, especially in seedling de-etiolation, and that its mRNA levels remained detectable in older seedlings and adult plants (Armstrong et al. 1995; Oosawa et al. 2000; Su et al. 2001; Frick et al. 2003).
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