HER2+ endometrioid endometrial cancer possesses distinct molecular and immunologic features associated with a more active immune microenvironment and worse prognosis

•HER2+ is present in 5.47% of endometrioid endometrial cancer (E-EMCA) and is associated with microsatellite stable disease.•HER2+ disease is a negative prognostic factor in E-EMCA.•HER2+ microsatellite stable E-EMCA displays increased TP53 mutations and reduced PTEN and CTNNB1 mutations.•HER2+ E-EMCA possesses many features of a more active immune microenvironment and MAPK pathway activation.