To dissect the interaction between serum TK1 activity in tumors with varying responses to palbociclib, we analyzed the TK1 data by tumor Ki-67 response category using the cut-off point of 2.7% (at which level complete cell cycle arrest was defined): anastrozole-sensitive (C1D1 Ki-67 ≤ 2.7%), palbociclib-sensitive (C1D1 Ki-67 > 2.7% but C1D15 Ki-67 ≤ 2.7%), and palbociclib-resistant (C1D15 Ki-67 > 2.7%). The gene discussed is MKI67; the disease is neoplasm.