Final answer:
Methylation of histone H3 at lysine 9 leads to chromatin condensation and repression of gene expression, which can affect cellular processes such as X-chromosome inactivation. Drugs that modify these epigenetic markers may influence gene expression to combat cancer.
Step-by-step explanation:
The effect of methylation of histone H3 at lysine residue 9 (K9) is significant in the regulation of gene expression. This methylation process leads to the condensation of chromatin, making it less accessible for transcription factors and thus, effectively repressing gene expression. As a result, genes in the methylated regions are typically not expressed. Conversely, hypo-acetylation of histones leads to a looser packing of nucleosomes, which can trigger inactivation of one of the X chromosomes, a process known as X-chromosome inactivation.
New drugs target these epigenetic modifications, aiming to decrease DNA methylation and prevent histone de-acetylation. This can re-activate repressed genes in tumor cells, potentially interfering with cancer cell growth and survival, offering a therapeutic strategy for cancer treatment.