ABCG2 and colorectal carcinoma: Heterogeneous ABCG2 expression was observed in some whole sections of CRC tissue samples from the 72 mCRC patients, as shown in Fig. 7, in which tumor cells in part of the tumor exhibited strong basolateral membrane staining, whereas tumor cells in another part of same tumor displayed considerably less intense staining, thus clearly demonstrating that tumor heterogeneity is a concern in some tumors.