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:coli cells, corresponding cDNA fragments were cloned into the expression vector pGEMEX-2Nde, which contained an isopropyl-2-thio-b-D-galactopyranoside (IPTG)–inducible promoter (Thöny et al. 1994).
Category: