= Solution
<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.<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]{ref}
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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