In addition to this, we observed that the chemo-resistance and P-gp expression of R-HepG2 were gradually ameliorated when DOX was removed from culture medium during passages (unpublished data), indicating the potential of R-HepG2 to differentiate into chemo-sensitive progenitor cells (i.e., heterogeneous differentiation) and thereby repopulating the tumor bulk during the breaks in chemotherapy. The gene discussed is PGP; the disease is neoplasm.