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:Single-molecule FRET observation and analysis were carried out by using the Cys-light kinesin monomer (349-residue with C-terminal His6 tag) with cysteines introduced into 215/342 or 43/342 residues as described (18), except that fluorescently labeled kinesins were immobilized on the glass surface via anti-His antibodies in the absence of
Category: