ESR1 and breast cancer: As demonstrated previously in NMIBC, the assessment of cytokeratin (CK) 5 and CK20 protein expression by immunohistochemistry correlates well with KRT5 and KRT20 mRNA expression but lacks prognostic value [21], which is in line with previous breast cancer studies investigating MKI67 (marker of proliferation Ki-67), ER (estrogen receptor), ERBB2 (Erb-B2 Receptor Tyrosine Kinase 2), and PR (progesterone receptor) mRNA and protein expression [37].