What are the different types of epigenetic modifications?

The three types of epigenetic modifications are DNA methylation, non-coding RNA, and histone modification. 

1. DNA methylation involves adding a chemical group to specific regions of DNA, which prevents proteins from accessing the DNA and reading the genes located there. This process effectively turns genes off. Conversely, demethylation removes these chemical groups, allowing proteins to access and read the genes, thereby turning them on. 
2. Non-coding RNA action plays a role in gene expression regulation by interacting with coding RNA. It can attach to coding RNA (along with specific proteins) to degrade the coding RNA, preventing protein synthesis. Additionally, non-coding RNA may interact with proteins to modify histones, and thus regulate gene expression by turning genes on or off. 
3. Histone modification refers to chemical changes made to histone proteins, around which DNA is wrapped. When histones are tightly packed, the DNA is less accessible to proteins that read genes; this results in gene silencing. Conversely, when histones are loosely packed, more DNA is exposed and accessible to gene-reading proteins, leading to gene activation.