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:2002) or by enforcing constitutive expression of Dnmt1 in cells (Feltus et al. 2003) or of the CpG-specific bacterial DNA methyltransferase SssI in isolated nuclei (Fatemi et al.
Category: