TK1 and neoplasm: As shown in Fig. 3 and Table 2, there was no statistically significant difference in TK1 activity between baseline and C1D1 following 28 days of anastrozole monotherapy (median serum TK activity was 46 versus 42.55 Du/L, respectively; p = 0.52), despite a significant reduction in tumor Ki-67 index, as well as, a reduction in tumor TK mRNA level.