The Southern blot is used to detect and identify certain $DNA$ sequences in a sample of bodily fluid. It uses single-stranded $DNA$ to search out their complementary strands. When a Southern blot is performed on $DNA$, the first step is digestion of $DNA$ with restriction enzymes. Restriction enzymes cut $DNA$ at known sequences, and produces $DNA$ fragments of a certain length. Once the $DNA$ is cut into pieces, scientists conduct electrophoresis to separate them by size.