MKI67 and breast cancer: The mean tumor volume in mice injected with MDA/LUC-shUGT8 cells was 11% (27.4 mm3, ±11.5) of the total control tumor volume (p<0.0001, two-way ANOVA test) To determine if the observed differences in tumor volumes were associated with in vivo proliferative potential and apoptotic properties of breast cancer cells, MDA-MB-231/LUC and MDA-MB-231/LUC-shUGT8 tumors were subjected, respectively, to staining with mAb directed against Ki67 antigen and TUNEL assay.