** Correspondence to: Laboratory of Cell Biology, Cancer Research Institute, Seoul National University College of Medicine, 103 Daehak-ro, Jongno-gu, Seoul 03080, South Korea.

Organoid culture technique has been taking center stage as a next-generation ex-vivo model due to advancement of stem cell research techniques. The importance of the laboratory-based ex vivo model has increasingly been recognized for recapitulating histological, and physioglocal conditions of in vivo microenviorment. Accordingly, the use of this technique has also broadened the understanding of intratumoral heterogeneity which is closely associated with varied drug responses observed in patients. Likewise, studies on heterogeneity within a single tumor tissue have drawn much attention. Here, we isolated 15 single clones from 4 tumor organoid lines from 1 patient at a primary passage from one patient. Each organoid line showed variable alterations in both genotype and phenotype. Furthermore, our methodological approach on drug test employing a high-throughput screening system enabled us to pinpoint the optimal time frame for anti-cancer drugs within a single tumor. We propose that our method can effectively reveal the heterogeneity of time-point in drug response, and the most optimal therapeutic strategies for individual patient.