Apparent stiffness of leukemic cells increases with progression of cell death. (A) A typical stiffness trace of a single M5 AML cell exposed to 1μM daunorubicin (circles). The apparent stiffness of a typical control cell remains relatively constant (triangles) and does not undergo apoptosis or cell death during the course of the experiment. Transition from open to filled shapres represents onset of cell death. Early apoptosis is defined as caspase 3 or 7 postivie staining and late apoptosis/dead is defined as Sytox Green (marker for cell membrane integrity loss) positive staining. (B) From the same patient sample, the average apparent stiffness of a population of late apoptotic/dead AML cells was significantly stiffer than early apoptopic cells and controls (n = 15, p< 0.05). Error bars are standard error.