MKI67 and cancer: However, in a pre-operative window trial of pictilisib and anastrozole vs. anastrozole alone, treatment with pictilisib preferentially reduced tumor Ki67 levels, a marker of proliferation, in luminal B rather than luminal A cancers.28 While these results would appear to oppose those presented in our study, there are likely confounding effects in this analysis; luminal A tumors typically have a very low basal proliferation rate, and are therefore much less likely to demonstrate significant reduction in Ki67 following therapy compared to luminal B tumors.