Expression of cyclin B1 in polyploid MKs. Cultures were performed as described in Fig 1. At day 8, cells were labeled by an anti-cyclin B1 MoAb (D and F) or an isotype control (FITC, FL1; C and E) and propidium iodide (FL3). Cells (10,000 8N cells) were acquired in a gate R1 (FL3A v FL3W; B) intersected with a gate R2 (FSCv SSC; A) to eliminate cell aggregates and cell debris. Ploidy was studied using simultaneously linear (FL3A; C and D) or logarithmic (FL3H; E and F) amplifiers. A specific labeling with the anti-cyclin B1 Moab is shown on (D) and (F) at the level of the 8N, 16N, and 32N peaks. No labeling is present in the S phases from 8N to 16N or from 16N to 32N.