Cells transfected with NRAS (K61) were not resulted in apoptosis. After 36 h of transfection with NRAS (K61), MDCK was changed into comet cells. Cell death detection kit for immunohistochemical detection and quantification of apoptosis at single cell level, based on labeling of DNA strand breaks (TUNEL technology): analysis by light microscopy. In positive control, cells were incubated with DNase I recombinant for 10 min to induce DNA strand breaks. Arrow tip marked the apoptosis cells. For the NRAS (K61) transfected cells, there were no bright brown particles.