KRT18 and breast cancer: In BC, the most commonly used assays include CYFRA 21.1—for CK19, tissue polypeptide antigen (TPA)—for CK8, CK18, CK19 and tissue polypeptide-specific antigen (TPS)—for CK8 and CK18 (Moar et al. 2023; Mirabelli and Incornoto 2013; Nicolini et al. 2015).