a Immunohistoschemistry analysis on resected specimen of GIST, which is clinically well controlled with imatinib. A small cluster of viable cells can be seen. HE Hematoxylin Eosin. b A small fraction of GIST-T1 cells enter into a dormant drug-tolerant “persister” state in response to ≥9 days of 2 μM imatinib treatment (right). Scale bars, 200 μm. c Cell-cycle assay of GIST-T1 parental cells (left) and persister cells (right). d Chemosensitivity assay (left) and cell proliferation assay (right) of parental cells, persister cells and regrown cells. Scale bars, 200 μm. Chemosensitivity was indicated by viability after treatment with imatinib for 72 h. e Persister cells showed regrowth after 9 days of “drug-holiday” of imatinib. Data represent mean ± SD (n = 6), *P < 0.05.