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:…previously identified POR genes, AtPORC is not expressed so strong in early seedling development, but its transcript levels increase in etiolated seedlings by illumination and are predominantly detectable in older seedlings and adult plants (Oosawa et al. 2000; Su et al. 2001; Frick et al. 2003).
Category: