With these anti-TK1 antibodies, TK1 protein concentration in serum (STK1) is now useful for prognosis [17,22,26,27], treatment monitoring [14,16,18,19,21–23], and discovery of recurrence [17–19,21,24] of various types of solid tumours, in addition to lymphoma and leukaemia, showing an increasingly important role of TK1 in the clinical setting. Here, TK1 is linked to leukemia.