Another study found that basic HIF-1α protein and BCRP mRNA and protein in AI (letrozole or exemestane)-resistant and HER2-transfected cells were higher than those in AI-sensitive HER2 parents under nonhypoxic conditions and BCRP mRNA in LTLTCa cells (AI-resistance breast cancer cells) treated with CoCl2 (HIF-1α stabilizer) increased by about two times compared with the control group. This evidence concerns the gene ABCG2 and breast cancer.