Overexpression of Meis1 and Pbx proteins induces apoptosis. Human Reh cells were transiently transfected with constructs expressing murine Meis1a or Meis1b or the murine Pbx proteins Pbx1b, Pbx3a, and Pbx3b by calcium-phosphate transfection. Cells were counted by trypan blue–dye exclusion assay at 1-day intervals after transfection. (A) Percentages of cell death were assayed at 1, 2, and 3 days after transfection for cells transiently transfected with Meis1a or Meis1b isoforms or with empty vector. Percentages of mock-transfected cultures were arbitrarily set to 0 to normalize data. Results are pooled means and standard deviations of 3 trials consisting of 3 replicate samples. (B) Percentages of cell death for cells transiently transfected with Pbx1b, Pbx3a, Pbx3b, or empty vector. (A-B) Transfection efficiency, 39%. (C-E) DAPI staining of transiently transfected cells. (C) Vector-transfected cells display uniform nuclear staining. (D-E) Murine Meis1b- and Pbx1b-transfected cells exhibit staining of condensed nuclear material, characteristic of apoptosis (arrows).