The most annoying thing about us biologists is that we keep using words that we don’t understand. “Epigenetics” is one of those that has drawn my attention for several years, as I explained in my last post. I suggested that the invasion started in 2001, the year that the histone code hypothesis was proposed by Thomas Jenuwein and David Allis in a seminal paper entitled Translating the Histone Code.

The histone code hypothesis was arguably the most influential concept of the last decade in molecular biology. Yet, most biologists would be hard pressed to say what the hypothesis is. All you have to do is read what Thomas Jenuwein and David Allis actually wrote. But believe it or not, this blog is one of the only places on the Internet where the histone code hypothesis is spelled out clearly. Most sources, including the Wikipedia article diverge substantially from the original statement, which is the following.

Distinct qualities of higher order chromatin, such as euchromatic or heterochromatic domains, are largely dependent on the local concentration and combination of differentially modified nucleosomes.

DNA in the nucleus comes in a structure called the nucleosome. The picture above is a molecular...