Accumulation of dGTP and induction of cell death during therapy. Blood samples were collected from all patients (except patient no. 1) at the indicated time points before and after start of therapy, and CLL lymphocytes were isolated. After methanol extraction, cells were processed to measure dGTP by DNA polymerase assay (A) and cell death by annexin binding procedure (B) as described in sections on measurement of dNTP pool and cell viability.