Flow cytometric cell cycle histogram. HUVECs treated with staurosporine (left panels) or kept in suspension with serum deprivation (right panels) for 4, 8, and 16 hours were permeabilized, fixed, treated with RNAse, and stained with PI. The region below the G_o /G₁ peak (designated A_o region) represents cells undergoing apoptosis-associated DNA degradation (hypodiploid DNA) and is expressed as a percentage of events with respect to the entire cell cycle.