A 3-D histogram plot of proliferating K562 cells (A) stained for cyclin B1 (FITC conjugated final antibody) and propidium iodide for DNA content. The majority of cells in G1 and S-phase have not stained positive for cyclin B1. An imposed G2M cell cycle arrest (1 μg ml^-1 etoposide for 24 h) (B) shows cells in G2-M staining strongly for cyclin B1.