Histones are highly conserved proteins that serve as the structural scaffold for the organization of nuclear DNA into chromatin. The four core histones, H2A, H2B, H3, and H4, assemble into an octamer (2 molecules of each). Subsequently, 146 base pairs of DNA are wrapped around the octamer, forming a nucleosome, the basic subunit of chromatin. Histone modifications regulate DNA transcription, repair, recombination, and replication. The most commonly studied modifications are acetylation, phosphorylation, methylation, and ubiquitination. These modifications can alter local chromatin architecture, or recruit trans-acting factors that recognize specific histone modifications (the "histone code" hypothesis). Trimethylation of histone H3 on Lys9 (H3K9me3) is one of the most highly studied epigenetic marks. H3K9me3 functions in the repression of euchromatic genes, and in epigenetic control of heterochromatin assembly, most likely via acting as a recognition motif for the binding of chromatin-associated proteins, such as Swi6 or HP1Alpha/Beta. The enzymes responsible for H3K9me3 formation are SUV39H1 and SUV39H2.