The DNA in a cell is usually wrapped around proteins called histones, which are packed tightly together to form a structure called chromatin . The main purpose of histones is to condense DNA and to regulate chromatin (and, therefore, to influence gene regulation). Histones can bear certain chemical marks, which are a result of chemical changes to the structure of the histone, also known as post-translational modifications. The modifications can affect gene expression through various mechanisms, and can either activate or repress DNA regions. These mechanisms presumably evolved along with the need for a system to control development in animals. Until recently, however, we have not known just how ancient these mechanisms of gene regulation are among animals.