FIG. 2.

The epigenetic seed and soil model. Individuals with different backgrounds, representing unique combinations of intrinsic and extrinsic factors, will have differing “epigenetic soil.” The “seed” represents the cellular signaling arriving at a gene promoter, either homeostatic signaling or signaling arising from an acute stimulus (eg, a toxicant exposure). In more responsive individuals, the epigenetic soil is more receptive to the incoming seed, resulting in increased gene transcription. Increases in gene transcription may either occur at baseline (a result of homeostatic signaling) or in response to an acute stimulus.