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:Also, we previously found that PFD decreases the content of mouse liver CuZn-SOD, mGSTA3, cytochrome P450 proteins (cyt P450) and CA III [47, 48].
Category: