Final answer:
Histone modification that leads to gene silencing involves epigenetic mechanisms such as histone deacetylation and DNA methylation which prevent transcription by compacting chromatin structure.
Step-by-step explanation:
The histone modification leading to the silencing or non-expression of genes is known as epigenetic regulation. This consists of adding methyl groups to DNA or modifying histones, particularly through methylation and deacetylation. In the context of cancer, genes are often silenced through the methylation of cytosine DNA residues in CpG islands of the promoter region and the deacetylation of histones surrounding this region. These epigenetic modifications compact the chromatin and prevent transcription factors from accessing DNA, thereby silencing the gene.
These changes do not alter the nucleotide sequence of DNA but rather affect the chromosomal structure. Regulatory proteins like histone acetyltransferases and histone deacetylases modify histones' tails thereby changing the accessibility of DNA for transcription. Since these histone and DNA modifications are reversible, they open pathways for potential therapeutic interventions in diseases like cancer where abnormal gene silencing occurs.