The intron splicing takes place within a large protein complex called a spliceosome, and these introns, the spliceosomal introns, are not assigned a group number. A spliceosome is made up of multiple specialized RNA-protein complexes called small nuclear ribonucleoproteins (snRNPs).

Each snRNP contains one of a class of eukaryotic RNAs, 100 to 200 nucleotides long, known as small nuclear RNAs (snRNAs). Five snRNAs (U1, U2, U4, U5, U6) involved in splicing reactions are generally found in abundance in eukaryotic nuclei.

In yeast, the various snRNPs include about 100 different proteins, most of which have close homologs in all other eukaryotes. In humans, these conserved protein components are augmented by more than 200 additional proteins.

Spliceosomes are thus among the most complex macromolecular machines in any eukaryotic cell. The RNA components of a spliceosome are the catalysts of the various splicing steps. The overall complex can be considered a highly flexible nucleoprotein chaperone that can adapt to the great diversity in size and sequence of nuclear mRNAs.