There can be different levels of histone acetylation as well as other types of modifications, allowing the cell to have control over the level of chromatin packing during different cellular events such as replication, transcription, recombination, and repair. Acetylation is not the only regulatory post-translational modification to histones that dictates chromatin structure; methylation, phosphorylation, ADP-ribosylation, and ubiquitination have also been reported. These combinations of different covalent modifications on the N-terminal tails of histones have been referred to as the histone code, and it is thought that this code may be heritable and preserved in the next cell generation.