The ribonuclease (RNase) protection assay provides a highly sensitive method for the detection and quantitation of specific RNAs from tissues and cells as well as for the analysis of mRNA and gene structure (1 ). This solution hybridization approach is at least 10-fold more sensitive than Northern blot analysis, and thus is useful for the evaluation of low-abundance mRNAs. Furthermore, the greater stability of the RNA duplex structure over RNA-DNA hybrids used in S1 nuclease protection assays provides the greatest sensitivity among the solution hybridization approaches. Structurally, these assays also provide a useful means of determining the size of specific exons (2 ,3 ) in a gene and for the quantitative analysis of specific alternative splicing events occurring in homogeneous tissues (seeNote 1 ), as well as the mapping of transcription start site (4 ,5 ).
