ABCG2 and colon adenocarcinoma: Overexpression of adenosine triphosphate-binding cassette (ABC) efflux transporters, such as ABCB1 (multidrug resistance protein 1 (MDR1)/permeability glycoprotein (P-gp)), ABCC1 (multidrug resistance-associated protein 1 (MRP1)) and breast cancer resistance protein ABCG2 (BCRP), is a key factor in PDT resistance in colon adenocarcinoma and lung carcinoma cells [14, 20, 51].