In particular, when methyl groups (made up of a carbon atom and three hydrogen atoms) are plastered on to a gene, they can stop that gene from being expressed altogether.
One example: The gene that makes the Epogen protein is expressed in only a few scattered tissues and is hard to detect by conventional gene-finding methods, which discover genes not by looking at DNA but by examining the RNA messengers that ferry genetic code between the DNA itself and the cellular machinery that make proteins.
These carrier screenings involve identifying individuals who carry one copy of a gene for a disease that requires two copies for the disease to be expressed in a child.
Rudolf Jaenisch. (Applause.) The 2010 National Medal of Science to Rudolph Jaenisch, Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology, for improving our understanding of epigenetic regulation of gene expression, the biological mechanisms that affect how genetic information is variably expressed.