Partitioning of cyclin A–expressing cells within the cell cycle. K562 cells were double stained for cyclin A expression (indirect FITC-staining) and for DNA content (propidium iodide [PI]). This was a test of our ability to simultaneously determine the expression of an intranuclear antigen by using indirect staining and DNA content. (Top) Flow histogram, PI staining on y-axis; FITC-staining on x-axis. Vertical line denotes 98%-negative isotype control gate. Note most negative cells at left edge of histogram. (Bottom) Cell-cycle distribution of cyclin A-negative cells (as represented by arrow connecting to histogram gate). (Middle) Cell-cycle distribution of cyclin A⁺ cells. G0/G1 and G2 DNA contents are highlighted in black; S-phase is highlighted with hash marks.