While human epidermal growth factor receptor 2/3 (HER2/3Total) were quantified by MS, RPPA further quantified several phosphorylated epitopes (pHER2Y1248, pHER2Y877, pHER3Y1289) with relevance to clinical trials investigating HER2-targeting agents in patients with USC and/or other endometrial cancer subtypes (NCT05256225, NCT04704661, NCT04486352). The gene discussed is ERBB2; the disease is endometrial cancer.