RNA duplexes containing the modified base 2-amino-adenine in place of adenine are stabilized through the formation of three hydrogen bonds in 2-amino A· U base pairs. Antisense 2′-O-alkyloligorlbonucleotide probes incorporating 2-aminoadenosine are thus able to efficiently affinity select RNP particles which are otherwise inaccessible. This has allowed the efficient and specific depletion of U5 snRNP from HeLa cell nuclear splicing extracts. U5 snRNP is shown to be essential for spliceosome assembly and for both steps of pre-mRNA splicing. The absence of U5 snRNP prevents the stabte association of U4/U6 but not U1 and U2 snRNPs with pre-mRNA.