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:In humans, however direct protein– protein interactions between Atox1 and MBDs are essential for the delivery of Cu to the MBDs (Hamza et al. 1999; Hussain et al. 2009; Larin et al. 1999; Walker et al. 2002, 2004; Yu et al. 2017b).
Category: