To confirm the role of copper ions in cytotoxicity of wedelolactone, breast cancer cells were transiently transfected with plasmid coding for human copper transporter CTR1. The transfection efficiency determined by flow-cytometry was 65.6% ± 3.7%, 43.6% ± 4.2%, and 46.6% ± 3.8% in MDA-MB-231, MDA-MB-468, and T47D cells, respectively (Figure S1). Here, SLC31A1 is linked to breast cancer.