Kaplan-Meier analysis of EFS for ALL patients with normal and elevated DHFR as a function of WBC. EFS data are shown for 17 patients with WBCs less than 50,000/μL (top panel) and 16 patients with WBCs exceeding 50,000/μL (lower panel) whose blasts were characterized by high (fluorescence ratio ≥2.4) or low (ratio <2.4) DHFR levels at diagnosis. As described in Materials and Methods, normal DHFR (fluorescence ratios of 1.6 to 2.4) was defined by 5 patients (BP6, 8, 13, 30, and 31) who survived at least 5 years without relapse. Experimentally measured DHFR values in excess of this range were considered to be elevated.