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:Nucleotide sequences in databases indicate that some genes in soil bacteria genomes were originated from integration of plant genetic material (Doolittle et al., 1990; Smith et al., 1992; Wakabayashi et al., 1986).
Category: