To evaluate the application of L4 and S21, we next examined whether these scFvs could detect endogenous nectin-4 expressed in human breast cancer cell lines, including MCF-7 (luminal type A), BT-474 (luminal type B HER2-positive), and MDA-MB-453 (triple negative) (10, 26, 35). Here, ERBB2 is linked to breast cancer.