**5. Conclusions**

In summary, our results show that ABCC1 plays a role in breast cancer proliferation, whilst ABCC4 has a greater role in cellular migration and invasion. It may well be that both transporters are important; their overlapping substrate specificity means they can likely compensate for each other. The mechanism by which these transporters (and others) are involved in the development and progression of breast cancer needs to be investigated further. It is key to know exactly how they are involved before they can be considered as targets for treatment.

**Supplementary Materials:** The supplementary materials can be found at http://www.mdpi.com/1422-0067/21/20/ 7664/s1. Figure S1: Expression of ABCB1/P-glycoprotein or ABCG2 in breast cancer cell lines. Figure S2: MK571 and Reversan affect the proliferation of breast cancer cells. Figure S3: Original, uncropped Western blots I. Figure S4: Original, uncropped Western blots II.

**Author Contributions:** Conceptualization, A.J.R. and F.G.L.; methodology, F.G.L., validation, F.G.L.; formal analysis, F.G.L., K.S., and A.J.R.; investigation, F.G.L. and K.S.; resources, A.J.R., J.E.B., and R.M.B., writing—original draft, F.G.L. and A.J.R.; writing—review and editing, K.S., R.M.B., and J.E.B., visualization, F.G.L. and A.J.R.; supervision, A.J.R., R.M.B., and J.E.B.; and funding acquisition, A.J.R., F.G.L., and J.E.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** F.G.L. was the recipient of an Aston University overseas excellence bursary. K.S. was funded by the Aston Medical School and University Hospitals Coventry and Warwickshire. Thanks to the Aston LHS joint research group fund.

**Acknowledgments:** Many thanks to Charlie Clarke-Bland for help with the Cell-IQ. The underlying data for this study can be found at the Aston Data Explorer Repository (https://doi.org/10.17036/researchdata.aston.ac.uk. 00000483).

**Conflicts of Interest:** The authors declare no conflict of interest.
