Within the confines of individual cells, minute changes in the concentration or spatial arrangement of molecular species can produce substantial effects. For example, a transcription factor equally prevalent in two isogenic cells might be bound to a promoter in one and unbound in another, subject to the dictates of statistical mechanics. Protein production would consequently begin in one cell and not the other, amplifying the fluctuation, and propelling each cell to a different fate. Identical genotype and an identical growth environment are thus insufficient to ensure that two cells will develop the same phenotypes. A major goal of our research has been to identify and differentiate between the myriad possible origins of this variability, understand which are biologically important, which are not, and to put firm numbers on each of them.