MKI67 and breast cancer: Likewise, aerobic exercise seems to affect certain variables associated with cell proliferation in a transgenic mouse model of breast cancer and N-methyl-N-nitrosourea (MNU)-induced mammary carcinogenesis in female rats, including the reduction in tumor size [32], the number of developed tumors [33], or the expression of proliferation markers, such as the Ki-67 nuclear protein [34].