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:…(Trueman et al., 1996); two NADH-NR, NR1 and NR2 (Long et al., 1992); nitrite reductase, NiR (Lahners et al., 1988); glutamate dehydrogenase, GDH1 (Sakakibara et al., 1995); four cytosolic GS, gln1, gln2, gln3, and gln4, plastidic GS, gln5 (Sakakibara et al., 1992a); and asparagine synthetase…
Category: