MKI67 and breast cancer: Further, Ki67—a marker of cell proliferation and cell progression—significantly decreased following exendin‐4 treatment as well, but when mice were treated with exendin‐4 and exendin, the reduction in Ki67 mRNA was critically enhanced with expression levels similar to control levels, further suggesting that exendin‐4 attenuates breast cancer growth via GLP‐1R and through inhibition of cell proliferation in vivo.