Hemophilia B (Christmas disease), is caused by factor IX deficiency.

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#Unit 3. Fundamental Processes

In Drosophila, an example of position effect variegation in the fly eye. 

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#Unit 3. Fundamental Processes

Some of the regions in the eye lack color, whereas others are red. This is because the white gene (required to develop red pigment) was inactivated by adjacent heterochromatin in some cells, but remained active in others.

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#Unit 3. Fundamental Processes

Su(var) for genes

whose products act to suppress variegation and E(var) for genes whose products enhance variegation.

active in others.

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#Unit 3. Fundamental Processes

Su(var) mutations lie in genes whose products are needed for the formation of heterochromatin such as HDAC, HMT.

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#Unit 3. Fundamental Processes

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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#Unit 3. Fundamental Processes

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).