ATP1B3 and breast cancer: In conclusion, the data reported here confirm that OU works as an “anti-estrogen-like” drug in all the ERα-positive BC subtypes (i.e., LumA, LumB, and metastatic BC) and further provide evidence that the use of OU and possibly of other CGs could be indicated for women with ERα-positive tumors expressing high levels of ATP1B3 and low levels of ATP1A1 and ATP1B1.