text: Many kinds of molecular-weight size markers exist, and each possess unique characteristics, lending to their involvement in a number of biological techniques. Selection of a molecular-weight size marker depends upon the marker type (DNA, RNA, or protein) and the length range it offers (e.g. 1kb). Before selecting a molecular-weight size marker, it is important to become familiar with these characteristics and properties. In a particular instance one type may be more appropriate than another. Although specific markers can vary between protocols for a given technique, this section will outline general markers and their roles.