Interference- Degree to which one crossover interferes with additional crossovers.

Interference = 1 − Coefficient of coincidence 

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E(var) mutations lie in genes whose products are needed to activate gene expression. They include members of the SWI/SNF complex

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HP1 (heterochromatin protein 1) is one of the most important Su(var) proteins. The HP1 chromodomain binds to histone H3 that is trimethylated at lysine 9 (H3K9me3).

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Mutation of a deacetylase that acts on H3K14Ac prevents the methylation at K9 because HDAC provide methylation sites for HMT.

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Methylation of H3K9 demarcates heterochromatin while H3K4 methylation demarcates euchromatin.

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Heterochromatin formation at telomeres and silent mating-type loci in yeast relies on an overlapping set of genes known as silent information regulators (SIR genes).

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Rap1 recruits Sir4 on acetylated histones, then recruits both its binding partner Sir3 and the HDAC Sir2. 

Sir3 and Sir4 then interact directly with histones H3 and H4. Once Sir3 and Sir4 have bound to histones H3 and H4, 

Sir2 deacetylates histones

Sir2-dependent deacetylation represses transcription.