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:…G19 directed against the TLA from chicken inner ear hair cells, mAb D10 directed against the avian hair-cell antigen, and antisera to protocadherin-15-CD1 (PB303) were characterized and validated as reported previously (Richardson et al., 1990; Ahmed et al., 2003; Goodyear and Richardson, 2003).
Category: