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:Several studies revealed redundant and essential roles of both, class I and II HDACs, in multiple levels of development and tissue patterning (De Rubertis et al., 1996; Shi and Mello 1998; Lagger et al. 2002; Brunmeir et al., 2009; Pillai et al., 2004).
Category: