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:For instance, the relatively unconserved 13.4 kb region between the 2.3 kb 5 enhancer and Ptf1a coding region contains sufficient information to direct expression specifically to Ptf1a-positive cells in the pancreas, and this expression is synergistic with the 2.3 kb element (Masui et al., 2008).
Category: