DNA exists as single molecules carrying genetic codes in individual cells.For this reason gene expression is stochastic.Single-molecule gene expression experiments in live single cell,when combined with single molecule enzymology in vitro, allowed quantitative and mechanistic interpretations of the fundamental processes of gene expression.The fact that there are 46 different individual DNA molecules (chromosomes) in a human cell dictates that genomic variation occurs stochastically and cannot be synchronized among cells.