440 Background: Cyclin E1 ( CCNE1) amplification ( CCNE1^Amp) drives chromosomal instability (CIN) and is associated with immune cell exclusion, liver metastasis, therapeutic resistance, and poor survival in gastric adenocarcinoma (GA). However, the clinical relevance of CCNE1^Amp in a larger real-world population is unknown. Therefore, we profiled the molecular landscape of CCNE1-amplified (amp) esophagogastric cancer (EGC) and explored treatment response outcomes. Methods: Tumors were analyzed by next-generation sequencing (NGS) of DNA (592 genes, NextSeq; WES, NovaSeq) and RNA (WTS, NovaSeq) (Caris Life Sciences, Phoenix, AZ). Deficiency in mismatch repair or microsatellite instability (dMMR/MSI-H) was tested by fragment analysis, IHC, and NGS. Tumor mutational burden (TMB) was measured by totaling somatic mutations per tumor (high>10 mt/MB). PD-L1 was tested by IHC (22C3, 1+). Real world overall survival (OS) was extracted from insurance claims and calculated from first of treatment to last contact using Kaplan-Meier survival curves for molecularly defined cohorts. Statistical significance was determined using chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q<0.05). Results: CCNE1^Amp (copy number, CN ≥6) was identified in 142/2276 (6.2%) esophageal adenocarcinoma (EA), 101/1449 (7.0%) esophagogastric junction carcinoma (EJC), 109/2607 (4.2%) GA, and 6/751 (0.8%) esophageal squamous cell carcinoma (ESCC) samples. Metastatic sites such as lymph node and liver showed higher frequency of CCNE1^Amp compared to primary sites. TMB, dMMR/MSI-H, and PD-L1 expression were similar in CCNE1-amp vs non-amp tumors. CCNE1^Amp was enriched in CIN type tumors (defined by increased TP53 and ERBB2 amp) and was associated with decreased mutation of CDKN2A and CDH1. In addition, CCNE1-amp tumors showed increased HER2 positivity/ ERBB2 amplification by IHC (14.3% vs 10.1%) or CN analysis (19.1% vs 10.3%). Differential gene expression analysis demonstrated increased liver X receptor/retinoid X receptor (LXR/RXR) pathway activation in tumors harboring CCNE1^Amp. While there were no differences in OS between CCNE1-amp vs non-amp GA, CCNE1^Amp was associated with worse OS after trastuzumab in the IHC HER2+ cohort (N=9 vs 28; HR (95% CI): 2.95 (1.18 – 7.34), p = 0.015). By contrast, there was a trend toward improved OS after immunotherapy in CCNE1-amp GA (N=9 vs 194; HR (95% CI): 0.34 (0.11 –1.07), p = 0.054). Conclusions: CCNE1^Amp is associated with a distinct molecular profile in EGC and resistance to HER2-targeted therapy. While CCNE1-amp tumors are thought to be generally “immune-cold,” CCNE1-amp GA demonstrated potentially improved outcomes with immunotherapy. Further investigation of CCNE1^Amp as a predictive biomarker is warranted, particularly as novel therapeutics selectively targeting CCNE1^Amp are under clinical investigation.