A nucleic acid "ladder" or molecular weight standard sample is a mix of DNA or RNA fragments with known lengths. It is used as a scale for determining the lengths of unknown nucleic acid fragments when performinggel electrophoresis experiments. The size of the fragments is determined by running a gel with the ladder in a well next to the samples with unknown lengths. The bands showing from the DNA ladder have predetermined lengths such as "100 bp", "500 bp" and others. If a band from an unknown sample has run the same distance in the gel as the 500 bp band, it can be assumed that the unknown fragment has a length close or equal to 500.

A gel electrophoresis is usually run with a "ladder" in one of the wells. Fragment lengths are given in base pairs. In this image also the concentration of the particular fragment in the "ladder" in displayed in ng/10uL.