embryo two copies of the X chromosome. Females, however, do not initially require both copies of the X chromosome as it would only double the amount of protein products transcribed as shown by the hypothesis of dosage compensation. The paternal X chromosome is quickly inactivated during the first few divisions. This inactive X chromosome (Xi) is packed into an incredibly tight form of chromatin called heterochromatin. This packing occurs due to the methylation of the different lysine residues that help form different histones. In humans X inactivation is a random process, that is mediated by the non-coding RNA XIST. Although methylation of lysine residues occurs on many different histones, the most characteristic of Xi occurs on the ninth lysine of the third histone (H3K9). While a single methylation of this region allows for the genes bound to remain transcriptionally active, in heterochromatin this lysine residue is often methylated twice or three times, H3K9me2 or H3K9me3 respectively, to ensure that the DNA bound is inactive. More recent research has shown that H3K27me3 and H4K20me1 are also common in early embryos. Other methylation markings associated with transcriptionally active areas of DNA, H3K4me2 and H3K4me3, are missing from the Xi chromosome along with many acetylation markings. Although it was known that certain Xi histone methylation markings stayed relatively constant between species, it has recently been discovered that different organisms and even different cells within a single organism can have different markings for their X inactivation. Through histone methylation, there is genetic imprinting, so that the same X homolog stays inactivated through chromosome replications and cell divisions. Mutations Due to the fact that histone methylation regulates much of what genes become transcribed, even slight changes to the methylation patterns can have dire effects on the organism. Mutations that occur to increase and decrease methylation have great changes on gene regulation, while mutations to enzymes such as methyltransferase and demethyltransferase can completely alter which proteins are transcribed in a given cell. Over methylation of a chromosome can cause certain genes that are necessary for normal cell function, to become inactivated. In a certain yeast strain, Saccharomyces cerevisiae, a mutation that causes three lysine residues on the third histone, H3K4, H3K36, and H3K79, to become methylated causes a delay in the mitotic cell cycle, as many genes required for this progression are inactivated. This extreme mutation leads to the death of the organism. It has been