Analysis of cloning efficiency
To analyze the cloning efficiency,cells were seeded as described in results and ,after 24 hours ,screened to count the number of wells with none ,one two or more than two cells. In order to verify that all the cells developed colonies,the cells were then incubated for one additional week, fixed with 70% ethanol, stained with 0.1% crystal violet in phosphate-buffered saline,washed four times in water and visualized in an inverted microscope. Routinely under our conditions,100% of the cells produced an equal amount of colonies.