As a potential target for AptaC2, the protein FXYD3 is a regulator of the activity of sodium/potassium-transporting ATPase (NKA), localized in the cytoplasmic membrane of epithelial cells, and is described as overexpressed in breast cancer, especially in primary tumors and in the ER+ subtype, urothelial carcinoma, and endometrial cancer [33,34,35,36,37]. The gene discussed is FXYD3; the disease is breast carcinoma.