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:…transcription depends on the level of activin receptor signaling in pituitary gonadotrophs, which in turn is modulated by the balance of locally produced activin and circulating levels of inhibin, an activin antagonist secreted from the gonads (Gregory and Kaiser, 2004; Bilezikjian et al., 2006).
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