Camptothecin sensitivity in human lymphoblasts. (A) EUFA867 lymphoblasts are sensitive to the topoisomerase I inhibitor camptothecin. Lymphoblasts were continuously exposed to different doses of camptothecin and cell growth was compared with untreated cells by cell counting. Wild-type, and FANCC- and FANCJ-deficient lymphoblasts were included as camptothecin-resistant controls. (B) Camptothecin sensitivity of EUFA867 lymphoblasts is due to a defect in FANCM. The FANCA defect in EUFA867 was corrected by stable transfection of flag-tagged FANCA (EUFA867+FANCA-flag); the FANCM defect was corrected by cell fusion with FANCA-deficient lymphoblasts HSC72 (EUFA867xHSC72OT fusion). Four independent cell fusions are depicted. (C) The C-terminus of FANCM is involved in camptothecin resistance. EUFA867 cells were stably transduced with FANCA and wild-type FANCM (FANCM) or a C-terminal deletion mutant of FANCM (delC-FANCM).