Abstract Gastric cancer (GC) is a major cause of global cancer mortality. Previous genomic studies have reported that several RTK-RAS pathway components are amplified in GC, with individual tumours often amplifying one component and not others (“mutual exclusivity”). Here, we sought to validate these findings for three RTK/RAS components (FGFR2, HER2, KRAS) using fluorescence in situ hybridization (FISH) on a series of gastric tumours, cell lines and patient-derived xenografts. Applying dual-colour FISH on 137 gastric tumours (89 FFPE surgical resections and 48 diagnostic biopsies), we observed FGFR2 amplification in 7.3% and HER2 amplification in 2.2% of GCs. GCs exhibiting FGFR2 amplification were associated with high tumour grade (p = 0.034). In FISH positive tumours, striking differences in copy number levels between cancer cells in the same tumour were observed, suggesting intra-tumour heterogeneity. Using a multicolour FISH assay allowing simultaneous detection of FGFR2, HER2, and KRAS amplifications, we confirmed that these components exhibited a mutually exclusive pattern of gene amplification across patients. The FISH data was also strongly correlated with Q-PCR levels and at the protein level by immunohistochemistry. Our data confirms that RTK/RAS components are mutually exclusively amplified in GC, and demonstrates the feasibility of identifying multiple aneuploidies using a single FISH assay. Application of this assay to GC samples, particularly diagnostic biopsies, may facilitate enrollment of GC patients into clinical trials evaluating RTK/RAS directed therapies. However, the presence of intra-tumour heterogeneity may require multiple biopsy samples to be obtained per patient, before a definitive diagnosis can be attained. Citation Format: Kakoli Das, Bavani Gunasegaran, Iain Beehuat Tan, Niantao Deng, Kiat Hon Lim, Patrick Tan. Mutually exclusive FGFR2, HER2, and KRAS gene amplifications in gastric cancer revealed by multicolor FISH. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4911. doi:10.1158/1538-7445.AM2015-4911