The human genome is about two meters (6.6 feet) long, yet it is folded inside a cell nucleus only about 10 micrometers in diameter. To fit into this tiny space, DNA is wrapped around histone proteins to form nucleosomes, which are further organized into chromatin. For decades, chromatin has often been described in two simple forms: euchromatin, which is active, open and accessible, and heterochromatin, which is more compact and repressed.