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:These strains were transformed with plasmid pME1983 (Gisi et al., 1999), a derivative of pTrc99A (Amersham) carrying the dcmA gene from Methylophilus sp. strain DM11 (Bader & Leisinger, 1994) under IPTG control, and the control plasmid pME1984 carrying the dcmA gene in antisense orientation to the P
trc
promoter.
Category: