Correlation between peak levels of TK⁺ cells and clinical response. TK⁺ circulating cells were monitored periodically by FACS analysis as ΔLNGFR-expressing cells. In patients with circulating TK cells detectable by FACS, peak of TK⁺ circulating cells was measured as the highest level of ΔLNGFR⁺ cells achieved in each patient (absolute numbers/μL). Evaluable patients are grouped as responders (CR+PR; ●60; n = 10) and nonresponders (NR; ○; n = 4). Medians are indicated. The difference between the 2 groups is statistically significant with P < .05 (2-tailed Wilcoxon test).