RNA: state memory and mediator of cellular phenotype.

It has become increasingly clear that the genome is dynamic and exquisitely sensitive, changing expression patterns in response to age, environmental stimuli and pharmacological and physiological manipulations. Similarly, cellular phenotype, traditionally viewed as a stable end-state, should be viewed as versatile and changeable. The phenotype of a cell is better defined as a 'homeostatic phenotype' implying plasticity resulting from a dynamically changing yet characteristic pattern of gene/protein expression. A stable change in phenotype is the result of the movement of a cell between different multidimensional identity spaces. Here, we describe a key driver of this transition and the stabilizer of phenotype: the relative abundances of the cellular RNAs. We argue that the quantitative state of RNA can be likened to a state memory, that when transferred between cells, alters the phenotype in a predictable manner.