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:In the P-element mediated transformation systems, transposons that contain proper cis elements are able to amplify at ectopic genomic loci, although levels of amplification are dramatically affected by chromosomal position effects (de Cicco and Spradling, 1984; Orr-Weaver and Spradling, 1986).
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