As might be expected, such a definition would include a wide variety of totally unrelated mechanisms, and indeed it does. The definition includes hereditary units that have their own DNA, the symbionts, and mitochondria. Cortical inheritance, a totally unrelated phenomenon that is based on the way the cortex of the cell grows and develops, is included. Serotype inheritance, completely unrelated to the other mechanisms, depends on the dynamics of protein synthesis. The remaining examples of epigenetics appear to be cases in which the phenotype is determined by the macronucleus, and the macronucleus is itself determined at the time of its formation. Only in the case of homology-dependent inheritance do we know the nature of these changes: deletion of whole regions of DNA. We know that in mating type inheritance, character differences are dependent on the differentiation of the macronuclei, but we do not know the chemical basis for these macronuclear changes. We might perhaps assume that they are due to deletions, as in the cases of homology-dependent inheritance. Indeed, the evidence is compatible with that interpretation, but we do not know whether it is true.