ABCG2 and colorectal carcinoma: A study by Wang et al. [31] investigated ABCG2 in 225 primary CRC tissues using IHC (antibody mouse monoclonal antibody, BXP-21, Abcam Company, Cambridge, MA, USA; a FOUR score system based on the intensity of immunostaining and the number of positive cells was used with separate scores for cytoplasmic and membrane immunostaining).