(4) Although combination of CDKis with endocrine therapy, such as fulvestrant, is a standard-of-care treatment, stronger activation of ER signaling in cancer cells with FGFR deregulation may require additional regimens to keep ER-RTK crosstalk under check for effective management of CDKi resistance in breast cancer with FGFR deregulation. This evidence concerns the gene ESR1 and breast carcinoma.