Figure 5 shows a case with ER-positive breast cancer on the upper outer quadrant of the left breast. PET images showed high homogeneous 16α-[18F]FES accumulation, and immunohistochemical findings of this case are high expression of ER. The other case with ER-negative breast cancer on the right axillary lymph node (Figure 6) showed no uptake on PET images, and immunohistochemical findings of this case are no expression of ER. This evidence concerns the gene ESR1 and breast cancer.