What does a methylated CpG island indicate?

A methylated CpG island specifically refers to the addition of methyl groups to cytosine bases within the context of a CpG dinucleotide. This biochemical modification occurs predominantly in regulatory regions of genes and is a crucial part of epigenetic regulation.

When cytosine residues in CpG islands become methylated, it typically alters the expression of nearby genes. This modification can lead to structural and functional changes in DNA, affecting how the DNA is packaged and ultimately how accessible it is to transcription machinery. Methylation often results in the repression of gene expression, which is why understanding the state of methylation in these regions is essential for studies in gene regulation and expression.

Thus, recognizing that methylated CpG islands signify the addition of methyl groups directly implies their role in modulating gene activity and highlights the significance of epigenetic modifications in regulating gene expression.