ABCB1 and neoplasm: We previously demonstrated the co-existence of two additional fractions each of drug transporter protein-expressing (ABCG2+/MDR1+) side population (SP1 and SP2) cells and non-expressing (ABCG2−/MDR1−) main population (MP1 and MP2) fractions (also known as non-side population (non-SP) cells), respectively, which incessantly replenished the other fractions during multiple cell culture passages and vividly reflected the potential for tumor plasticity [19,23].