The electrophoretic mobility shift assay (EMSA) is a widely used technique to study DNA–protein interactions. In EMSA, a labeled DNA probe (or RNA probe) is incubated with a purified protein or a protein-containing cell extract (such as nuclear extract). The DNA–protein binding alters the mobility of the DNA during electrophoresis, resulting in a shift in the migration of the complex compared to free DNA. This shift, known as a “band shift” or “gel retardation,” allows the detection and analysis of specific DNA–protein interactions. Nonradioactive EMSA typically uses fluorescent dyes or other detectable markers for visualization.