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:In addition, although the C-terminal cleavage fragment activates cytokine production elicited by CpG-DNA or NF-kB signaling, the levels of TNF-a production or NF-kB activation seem to be lower than those seen with full-length TLR9 (10, 19).
Category: