Correlations were identified between the markers that determine the molecular subtype of BC: percent of cells expressing ER and percent of cells expressing the PR positively correlated with each other (R = 0.63; p = 0.000); Ki-67 and HER-2 demonstrated a weak positive correlation (R = 0.15; p = 0.04); percent of ER and Ki-67 expressing cells revealed a negative correlation (R = −0.39; p = 0.000); percent of PR and HER-2 expressing cells showed a weak negative correlation (R = −0.23; p = 0.01). This evidence concerns the gene MKI67 and breast cancer.