In vitro IR-induced response in primary ALL reflects clinical response. A total of 7% of patients with apoptosis-sensitive ALL are PERs exhibiting more than 25% of leukemic blasts in bone marrow on the days 8/15 of induction treatment. In contrast, 38% of apoptosis-resistant patients are PERs. A total of 21% of apoptosis-sensitive samples are MRD⁺ (0.01% or more blasts in bone marrow on day 28 of induction treatment) by FACS analysis. In comparison, 59% of patients with apoptosis-resistant ALL are MRD⁺. A total of 19% of apoptosis-sensitive patients and 69% of apoptosis-resistant patients exhibit MRD positivity as measured by molecular MRD analysis.