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:development of genetic tools for species within Planctomycetes [71] and Verrucomicrobia [72] makes this hypothesis testable in the future using targeted mutagenesis, immunoelectron microscopy and other approaches requiring genetic tools.
Category: