Abstract It is essential to have appropriate preclinical models for developing new drugs for gastric cancer (GC). Cell lines have been the most commonly used model for this purpose. However, the numbers of gastric cancer cell lines established and available for this purpose are limited and not enough to deal with variety of gastric cancer molecular subtypes such as recently proposed by The Cancer Genome Atlas (TCGA). In addition, patient-derived xenograft (PDX) models have been considered more appropriate models for evaluating the efficacy of new drugs in vivo. For contributing to develop new drugs as well as to seek more effective biomarkers, we have been conducting an intramural project, called DEF study, for establishing new GC PDXs as well as new GC cell lines. So far, 30 multi-passage-competent new PDXs have been successfully established from surgically removed gastric cancer tissues or ascites. Twenty-two (22) new GC cell lines have also been established from PDX tissue (16 lines) or directly from ascites (6 lines). All GC PDXs have been examined by immunohistochemistry for HER2, EGFR, and Met expressions, and we identified 5 cases overexpressing HER2 (corresponding to Score3+) and 1 case overexpressing both EGFR and MET. Comprehensive gene mutation analysis has been performed in 21 PDX tissues, and repetitive mutations (SNV/Indel) have been observed in ERBB2, ERBB3, BRAF, APC, ARID1A, and p53. Copy number alternations have also been identified; examples include KRAS and ERBB2 for amplification and PTEN and CDKN2A for deletion. Citation Format: Takeshi Kuwata, Kazuyoshi Yanagihara. Establishment of novel gastric cancer PDX models and cell lines for developing new therapeutics [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A008.