KIT and gastrointestinal stromal tumor: To test the effects of drug withdrawal and rapid drug alternation on heterogeneous GIST populations, we established polyclonal co-cultures of GIST cell lines containing clinically relevant KIT mutations: exon 11 mutation alone (GIST-T1), exon 11 primary mutation in cis with exon 13 secondary mutation (GIST430/654), and exon 11 primary mutation in cis with exon 17 secondary mutation (GIST-T1/820), modelling the polyclonal imatinib-resistance heterogeneity observed in many GIST patients.