Background: Adenocarcinoma of esophagus (EAC) and GEJ is the fastest rising cancer in the US. The outcomes are extremely poor with median overall survival (OS) being 12 mo in patients (pts) with metastatic disease. The standard first line treatment for metastatic EAC is platinum-based regimen with median progression free survival (PFS) of 6 mo. Second line options are associated with limited efficacy. An analysis of TCGA has shown 40% of EAC harboring abnormalities in HR genes, most likely resulting from chronic acid reflux induced DNA damage. HR dysregulation is commonly associated with high LOH. Sensitivity to PARP inhibition has been shown to be a surrogate for HR defects or BRCAness phenotype. Clinically PARP inhibitors have shown activity in HR defective prostate and ovarian cancers. These findings provide the basis for this study. Methods: Pts with metastatic esophageal/GEJ/proximal gastric adenocarcinoma, previously treated with 1 line of platinum containing chemotherapy, and harboring high LOH and/or deleterious alteration(s) in HR genes (BRCA1/2, PALB2, ATM, BARD1, BRIP1, CDK12, CHEK2, FANCA, RAD51, RAD51B, RAD51C, RAD51D, RAD54L, NBN, ARID1A, GEN1) are eligible for this study. Pts can be prescreened at the time of diagnosis of locally advanced or metastatic disease by genomic analysis of the most recent available tumor tissue. Pts will receive oral niraparib until disease progression or unacceptable toxicity. Primary objective is response rate (RR). Secondary objectives are safety and tolerability, progression free survival (PFS), and disease control rate (DCR). Exploratory objectives include correlation between high LOH and response to niraparib, mechanisms of resistance to PARP inhibition, EZH2 expression and its correlation with response and resistance to PARP inhibition, and analysis of germline HR gene mutations and correlation with response to niraparib. Estimated sample size is 43. The study has recently opened to accrual at Indiana University with intended collaboration with 2 additional sites. Clinical trial information: NCT03840967