Epigenetics is the buzz word prominent in the current issue of The Scientist (see "Researchers Focus on Histone Code") and a recent theme issue of Science (August 10, 2001). The term was introduced by Conrad H. Waddington in 1942.1 To paraphrase an erudite epistolary exchange in Science, he is said to contrast genetics with epigenetics, the study of the processes by which genotype gives rise to phenotype. In 1942 we had barely any clue as to what those processes are, so "epigenetic" had no connotation of the underlying chemical mechanism, whatever it was that modulated cell differentiation.

In 1994, as cited in the same issue of Science, Robin Holliday voiced a commonly apprehended drift in meaning, and redefined epigenetic as "Nuclear inheritance which is not based on differences in DNA sequence." These two memes are freely circulating and can cause muddle or mischief mainly when...

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