Gastrointestinal stromal tumor (GIST) is often driven by oncogenic variations in KIT. While some patients respond to the usual starting dose of imatinib, approximately 10% to 15% of patients harbor a KIT exon 9 variation and require high dose imatinib for clinical benefit.11 However, only 15% of patients with GIST undergo variation profiling before treatment initiation.13 This economic evaluation is the first, to our knowledge, to report a Markov model assessing the cost-effectiveness of using TGT to tailor imatinib therapy in patients with metastatic GIST. This evidence concerns the gene KIT and gastrointestinal stromal tumor.