In vitro accumulation of dGTP and induction of cell death. Primary CLL cells from 6 patients were incubated without drug or with 2μM forodesine and 20μM dGuo for 24 hours, and the nucleotides were extracted, and dGTP was measured by DNA polymerase assay in untreated () or forodesine and dGuo–treated () cells (A). Data from Figure 4A are plotted with data from Figure 5A to compare in vivo () and in vitro () accumulation of dGTP (B). In the same cells, cell death was analyzed at 24 hours by Annexin positivity procedure as described in “Measurement of cell viability” in untreated () and forodesine and dGuo–treated () cells (C).