ESR1 and breast cancer: Among ER+ breast cancers, 15% harbored FGFR1 gene amplification and/or mRNA overexpression whereas 2.5, 1.9, 2.4, and 1.7% harbored amplification and/or mRNA overexpression of CRKL, HCK, FRK, and FGR, respectively (Fig. 2a).