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:AChE is required for endoderm adhesion to fibronectin
In vitro, AChE binds to extracellular matrix (ECM) ligands, promoting cell-
substrate adhesion (Johnson and Moore, 1999; Johnson and Moore, 2004; Johnson et al., 2008a).
Category: