The 123 bp DNA Ladder is suitable for sizing double-stranded DNA from 123 to 3,075 bp. Prepared from a plasmid containing repeats of a 123-bp DNA fragment (1), the ladder consists of 34 fragments ranging in length from 123 to 4,182 bp. Up to 25 separate bands may be clearly visualized on 0.7% to 1.5% agarose gels after ethidium bromide staining. The ladder is also useful on 4% to 8% polyacrylamide gels. This ladder can be stained with ethidium bromide or radiolabeled using T4 polynucleotide kinase, T4 DNA polymerase, DNA polymerase I, or the large fragment of DNA polymerase I (Klenow fragment).