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:A subclone named M5 with enhanced metastatic abilities in liver and a subclone named SCP 51 with enhanced metastatic abilities in liver and lymph node were isolated by in vivo selection of SW480 cells through a process described in our previous studies [18-20].
Category: