ABCG2 and immune system disorder: In order to evaluate the tumorigenicity of ABCG2+ versus ABCG2− cells, we inoculated subcutaneously 8 × 103, 4 × 104, 2 × 105, 1 × 106, and 5 × 106 ABCG2+ SMMC-7721 cells and the same amount of ABCG2− cells into immunodeficiency mice, respectively.