ABCG2 and squamous cell carcinoma: For example, Chen et al [21] evaluated gefitinib efflux by collecting conditioned media from human epidermoid carcinoma A431 cell clones expressing different level of ABCG2, by exposing MDA-MB-468 cells to these media, and finally by evaluating the inhibition of EGFR Tyrosine1068 phosphorylation.