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:Moreover HDAC1 is not crucial for enzymatic function of HDAC2 in embryonic stem cells, however overexpression of HDAC2 cannot compensate for the loss of HDAC1 (Lagger et al., 2002).
Category: