Minute-size fractions achieved in two-dimensional electrophoresis, IEF and PAGE with SDS, and bands from electrophoresis of nucleic acids are detected differently than protein electrophoresis fractions. Labeled polypeptide probes are used to detect these proteins; labeled single-stranded nucleic acid fragments are used for the detection of nucleic acids. Each probe is made with a label designed to generate a detectable signal. The label is bound to a probe and a system is created such that the signal is visualized when the probe is bound to the target.

The most common labels are radioactive isotopes and fluorescence dyes. Chemiluminescence and color or ultraviolet absorbance are also used.