Molecular-weight size markers can be broken up into two categories: molecular weight markers vs. molecular ladder markers. Markers are either stained or unstained, and depending on the circumstance, one may be more appropriate than another. Molecular-weight size markers can also be biochemically altered. The conjugation with biotin is the most common. Molecular-weight size markers are most commonly used in SDS-polyacrylamide gel electrophoresis and western blotting.
With all the different types and uses of molecular-weight size markers, it is important to choose the appropriate protein standard. Besides the most common use, as a way to calculate the molecular weight of the samples, other uses include allowing visual evidence of protein migration and transfer efficiency and are sometimes even used for positive control.