Intrinsic gene-expression noise is variability from the random reaction events of the expression process itself, conditional on the cell's shared state: transcript production and removal, translation events and protein loss. It remains possible in nominally identical cells in the same environment and is particularly visible when molecule numbers are small.
Extrinsic gene-expression noise comes from fluctuations in shared cellular conditions, such as cell size, growth state, expression machinery abundance or environmental signals. It can correlate otherwise distinct expression processes. In a matched two-reporter measurement, reporter differences reveal an intrinsic component while correlated cell-to-cell changes reveal a shared component, after reporter-specific measurement noise is accounted for.[ref]
Stochastic Gene Expression in a Single Cell by Michael B. Elowitz and others. Matched fluorescent reporters separate within-cell reporter differences from correlated cell-to-cell variation
Intrinsic fluctuations can be beneficial when a population gains a diversity of temporary physiological states, or when a noisy threshold crossing helps a cell explore or select a state. They can be detrimental when an essential product transiently falls too low, when tightly coordinated components become imbalanced, or when a regulatory threshold is crossed at an inappropriate time. These are context-dependent effects: noise is not automatically adaptive, and increasing a Fano factor is not synonymous with improving a cellular function.
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