text: Note: These lists are illustrative, not exhaustive, and identification of noise sources is an active and expanding area of research.
  ;Intrinsic noise
  * Low copy-number effects (including discrete birth and death events): the random (stochastic) nature of production and degradation of cellular components means that noise is high for components at low copy number (as the magnitude of these random fluctuations is not negligible with respect to the copy number);
  * Diffusive cellular dynamics: many important cellular processes rely on collisions between reactants (for example, RNA polymerase and DNA) and other physical criteria which, given the diffusive dynamic nature of the cell, occur stochastically.
  * Noise propagation: Low copy-number effects and diffusive dynamics result in each of the biochemical reactions in a cell occurring randomly. Stochasticity of reactions can be either attenuated or amplified. Contribution each reaction makes to the intrinsic variability in copy numbers can be quantified via Van Kampen's system size expansion.
  ;Extrinsic noise
  * Cellular age / cell cycle stage: cells in a dividing population that is not synchronised will, at a given snapshot in time, be at different cell cycle stages, with corresponding biochemical and physical differences;
  * Cell growth: variations in growth rates leading to concentration variations between cells;
  * Physical environment (temperature, pressure, ...): physical quantities and chemical concentrations (particularly in the case of cell-to-cell signalling) may vary spatially across a population of cells, provoking extrinsic differences as a function of position;
  * Organelle distributions: random factors in the quantity and quality of organelles (for example, the number and functionality of mitochondria) lead to significant cell-to-cell differences in a range of processes (as, for example, mitochondria play a central role in the energy budget of eukaryotic cells);
  * Inheritance noise: uneven partitioning of cellular components between daughter cells at mitosis can result in large extrinsic differences in a dividing population.
  * Regulator competition: Regulators competing to bind downstream promoters can cause negative correlations: when one promoter is bound the other is not and vice versa.
  Note that extrinsic noise can affect levels and types of intrinsic noise: for example, extrinsic differences in the mitochondrial content of cells lead, through differences in ATP levels, to some cells transcribing faster than others, affecting the rates of gene expression and the magnitude of intrinsic noise across the population.