ABCG2 and colorectal carcinoma: Finally, to evaluate differences between ABCC4 and ABCG2 mRNA expression and response to the main anti-CRC chemotherapy regimens, two datasets containing mRNA expression levels of CRC patients treated with FOLFIRI (GSE62080 consisting of n = 21; 12 resistant and 9 sensitive) and FOLFOX (GSE83129 consisting of n = 23; 12 resistant, 21 sensitive) were downloaded from the GEO database.