MODEL RELEASED. Western blot technique. Female researcher examining a Western blot film. This technique uses enzymes to fragment DNA (deoxyribonucleic acid), and the fragments are then separated in an electrophoresis gel. The separated fragments are then blotted" onto a nitrocellulose filter and marked with radioactive probes. The probes indicate the presence of specific genetic sequences (lengths of DNA that code for proteins). A sheet of photographic film is then laid over the filter. The radioactivity darkens the film, which reveals the position of genes in the DNA (seen here)."