MKI67 and breast cancer: In contrast to the previously discussed results, CXCR4 gene expression is increased in a transgenic mouse model of breast cancer with access to a wheel, compared with nonrunners, although pulmonary metastatic foci and the proliferative index, as indicated by morphometric analyses and measurement of Ki-67 levels, respectively, show no differences between runners and nonrunners, and no difference has been observed in the expression of CXCL12 gene [32] (Figure 2).