INTRODUCTION Alkaline agarose gels are run at a pH that is sufficiently high to denature double-stranded DNA. The denatured DNA is maintained in a single-stranded state and migrates through the alkaline gel as a function of its size. Alkaline agarose gels are used chiefly to measure the size of first and second strands of cDNA (Construction of cDNA Libraries Stage 1: Synthesis of First-strand cDNA Catalyzed by Reverse Transcriptase) and to analyze the size of the DNA strand after digestion of DNA-RNA hybrids with nucleases such as S1.