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:Mutant and WT hClC-1 channels were transiently expressed in HEK293T cells and examined typically 2 days after transfection with a calcium phosphate precipitation method (Fahlke et al. 1997a).
Category: