ABCB1 and cancer: However, acquired or intrinsic resistance occurs in about 40% of breast cancer patients due to three major resistance mechanisms [6]: Overexpression of multidrug resistance protein 1 (MDR-1), the occurrence of so called cancer stem cells (CSCs) as well as especially in this breast cancer subtype the ligand independent activation of ERα via different Receptor Tyrosine Kinases (RTKs) [7–9].