Inhibition of CML progenitor cell growth following exposure to As₄S₄ and/or imatinib. Patients at CP: n = 13 (except that the number of 0.1 μM imatinib/0.5 μM to 2 μM As₄S₄ combination groups was 8). Patients at AP: n = 6. Controls: n = 8 (except that the number of 0.5 μM imatinib/0.5 μM to 2 μM As₄S₄ combination groups was 7). +, additive; *, synergism. Error bars indicate 1 SD.