ER+ tumor explants treated with progesterone (10 nM) but not estrogen (1 and 10 nM) had a significantly higher percentage of Ki67-positive cells (a marker of cell proliferation), compared to vehicle treatment (P = 0.006, ANOVA with TukeyHSD posttest; n = 6) (Fig. 3a). Here, MKI67 is linked to neoplasm.