Coexpression of HoxA9 abrogates Meis1a-mediated but not Pbx1b-mediated apoptosis. (A-B) Effect of coexpression of murine HoxA9 with murine Meis1a (A) or murine Pbx1b (B). Total amounts of DNA were kept constant by transfecting empty vector with Meis1a- or Pbx1b-transfected cultures. Results are pooled means and standard deviations of 3 separate experiments consisting of 3 replicate samples. (A-B) Transfection efficiency, 39%. (C) Effects of increasing amounts of HoxA9 on Meis1b-transfected cells. Cells were transiently transfected with 5 μg murine Meis1b DNA alone or with 1 to 5 μg murine HoxA9 DNA. Total amounts of transfected DNA were kept constant using empty vector with single transfections. Transfection efficiency, 35%. (D) Resistance of Meis1b+HoxA9–expressing cells to induction of apoptosis by etoposide. Cells were transiently transfected with empty vector or murine Meis1b or were cotransfected with murine Meis1b and murine HoxA9. After 24 hours, 17 μM etoposide was added to each culture. Cell death was assayed at 1-day intervals after etoposide treatment. Transfection efficiency, 37%