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:More informative for the study of midbrain development was the discovery that a subtle shifting of Otx2's caudal expression boundary effects mDA neuron population size; a caudal shift results in an increase in population and a rostral one a reduction [19-21].
Category: