Nuclease protection assays (S1 nuclease protection and RNase protection) are extremely sensitive procedures for detection and quantitation of mRNA species in complex mixtures of total cellular RNA. These assays are well suited for mapping positions of external and internal junctions in RNA, such as transcription initiation and termination sites and intron/exon boundaries, and to discriminate between closely related targets by using probes designed to span the regions where the related genes differ the most. Also, because the size of the probes used in nuclease protection assays is a variable chosen by the investigator, probes may be designed to protect fragments of different sizes. This feature permits the simultaneous analysis of several different mRNAs in the same total RNA sample. In this unit, a method is included for RNase protection of target mRNA sequences, including hybridization of the probe to the target sequence, details of the actual protection assay, and detection of reaction products. An alternative method is provided for performing the RNase protection assay on a microvolume scale, which is useful when there are many samples to be analyzed. Support protocols describe synthesis and gel purification of labeled RNA probes; preparation of RNase-free yeast RNA, which acts as an aid in the quantitative precipitation of newly synthesized probe; and quantitation of target mRNA. A method describing S1 nuclease protection of target mRNA using either RNA or DNA probes is also included. Additional support protocols provide instructions for the preparation of radiolabeled DNA probes by primer-extension of double-stranded plasmid or PCR product using Klenow fragment of E. coli DNA polymerase I or Taq or Tth polymerase in a thermal cycler. Another radiolabeling method details 5' end labeling of oligodeoxynucleotides and oligoribonucleotides using T4 polynucleotide kinase. Additionally, a method is described for mapping transcription start sites using the S1 nuclease protection assay.