PGP and in situ carcinoma: To understand the role of secreted signals from A549-PacR cells with multiple acquired cross-resistance in promoting P-gp-associated MDR, we directly harvested CM from A549-PacR/5-FU, A549-PacR-/DCT, and A549-PacR-/CIS cells followed by individual or combination transient gene or siRNA transfections (see scheme in Figure 5J).