ABCG2 and neoplasm: They showed that CD9 plays a crucial role in the interaction between BMMSCs and HCC1806 within the tumor microenvironment by regulating the expression of tumor resistance-related proteins such as CXCL12, CCL5, CCR5, and BCRP, ultimately leading to an enhancement of drug resistance in DP-HCC1806:BMMSCs (Ullah et al., 2019).