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:As mentioned above, MAPT gene exon 10 alternative splicing yields comparable levels of the 3R and 4R tau isoforms.(6) The mutations tested in cells (11C, 12U, 13G, DD-PAC and 16U) increased the quantity of 4R isoforms by 20.
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