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:PRR5 is one of the less studied members of the PRR gene family, and all data collected to date on this gene pertain only to mRNA variation and the analysis of mutant and overexpression phenotypes (Matsushika et al., 2000; Sato et al., 2002; Eriksson et al., 2003; Michael et al., 2003).
Category: