CDK4 and breast cancer: As opposed to other histologies, point mutations, fusions, nuclear FGFR expression and FGF ligand amplification are initially infrequent in BC, however their clonal acquisition after treatment does play a role in ER+ BC, especially after treatment exposure with endocrine therapy (ET) and Cyclin–dependent kinase 4/6 inhibitors (CDK 4/6 inhibitors) (66, 67).