Vertebrate cells possess a protein that binds to clusters of 5-methylcytosine and ensures the gene will stay in the off position. this control of gene regulation is a result of

Methylation

Explanation:

The gene regulation is the important phenomena for the proper physical expression and metabolic function of the body. The gene may gets activate or inactivate depending upon the type of modification.

The acetylation modification increases the expression of gene. The methylation modification is done by the enzyme methyl transferase. The metylation at specific lysine residues result in the decrease or off gene expression.

Thus, the answer is methylation.