DNA contains the blueprints for all the proteins in the body. DNA is wound up in tight coils and when a cell wants to make a protein, these coils unravel to expose the instructions for making that protein. The degree to which DNA stays tightly coiled or loosens up is influenced by whether or not chemicals called "methyl groups" are attached to specific parts of the DNA.
The researchers hoped to find out if DNA methylation is different between twins when one twin has lupus and the other does not.
Read the research summary, as well as the original abstract.