Molecular weight size marker

A molecular weight size marker is used to identify the approximate size of a molecule run on a gel electrophoresis. This marker can be composed either of different proteins of known size, used in a Western Blot, or can be a nucleic acid that has been digested with a restriction enzyme into fragments of known size.

A commonly used DNA molecular weight marker is the genome of the Lambda phage following digestion using the restriction enzyme HindIII. This produces an array of fragments ranging from 125 to 23,130 base pairs. (Actual fragment sizes are: 23130, 9416, 6557, 4361, 2322, 2027, 564, and 125 bp.)

When used in gel electrophoresis, markers effectively provide a logarithmic scale by which to estimate the size of the other fragments (providing the fragment sizes of the marker are known).