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:Cx43 expression is highly sensitive to ionizing radiation, as shown by in vitro (Azzam et al., 2003) and in vivo studies (Kasper et al., 1996; Liu et al., 1997; Amino et al., 2006).
Category: