As mentioned above the histone tails have been shown to directly interact with the DNA of the nucleosome. Each histone in the octamer has an N-terminal tail that protrudes from the histone core. The tails play roles both in inter and intra nucleosomal interactions that ultimately influence gene access. Histones are positively charged molecules which allow a tighter bonding to the negatively charged DNA molecule. Reducing the positive charge of histone proteins reduces the strength of binding between the histone and DNA, making it more open to gene transcription (expression). Moreover, these flexible units direct DNA wrapping in a left-handed manner around the histone octamer during nucleosome formation. Once the DNA is bound the tails continue to interact with the DNA. The parts of the tail closest to the DNA hydrogen bond and strengthen the DNA's association with the octamer; the parts of the tail furthest away from the DNA, however, work in a very different manner. Cellular enzymes modify the amino acids in the distal sections of the tail to influence the accessibility of the DNA. The tails have also been implicated in the stabilization of 30-nm fibers. Research has shown removing certain tails prevents the nucleosomes from forming properly and a general failure to produce chromatin fiber. In all, these associations protect the nucleosomal DNA from the external environment but also lower their accessibility to cellular replication and transcriptional machinery.