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 GABAergic neuronal specification in the CNS. Intuitively, feedforward loops should maintain levels ad infinitum, and this is indeed the case for Pit-1 in the pituitary, Barhl1 in the inner ear and CNS, and Ptf1a in the pancreas (Chen et al., 1990; Chellappa et al., 2008; Masui et al., 2008).
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