DNA contains the instructions for building all parts of the body, but DNA is only half the story. The DNA in our body is spooled around proteins called histones, which package the cellular genome. Modification of the histones allows DNA to unwind and the instructions to be "read" and subsequently, the genes to be expressed.
Both the DNA and histones are covered with chemical tags, a second layer of structure called the epigenome. It tightly wraps inactive genes, making them unreadable. It relaxes active genes making them easily accessible. The DNA code remains fixed for life, but the epigenome is flexible.
The epigenome adjusts specific genes in our genomic landscape in response to our rapidly changing environment. Thereby, expression of genes can be affected by various factors: lifestyle, nutrition, chemicals and stressors in the environment, as well as dynamics in internal processes and functions of the body.