Correlation of DNA-PK activity with CLB sensitivity as determined in vitro. CLL lymphocytes were screened for CLB sensitivity in vitro by using the MTT assay as described in Materials and Methods, and the IC₅₀ for this drug was compared with DNA-PK activity. Each point represents the result from an individual patient's sample. (□), Lymphocytes obtained from untreated patients; (▪), lymphocytes obtained from treated-resistant patients. (Four patients were not included in this analysis bacause in vitro sensitivity to melphalan was done instead of chlorambucil.)