ESR1 and breast cancer: Furthermore, because different combinations of drugs (i.e., AZD and Prexa but not GDC with Tam only in LumA BC cell lines; MK, GDC, and Prexa with Abe and MK and Prexa with Palbo in ET-resistant LumA cells) have synergistic effects in different cell lines, our analyses unveil a potential for personalization of co-treatments with CHK1 inhibitors in different kind of ERα-positive BC cell lines.