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:CRISPs are single-polypeptide proteins with molecular weights of ;20 to 30 kDa (Yamazaki and Morita 2004, 2007) and are known to have a high degree of amino acid sequence similarity and a highly conserved specific pattern of 16 cysteine residues.
Category: