Finally, in PIGPCs with ABCG2 or empty control expression under Dox control, the addition of Dox significantly reduced sphere formation in Tet-ABCG2 SP but not Tet-ABCG2 MP, Tet-Empty SP or Tet-Empty MP cells (Fig. 2d), again suggesting an ABCG2-specific regulation of tumor cell self-renewal. The gene discussed is ABCG2; the disease is neoplasm.