A spliceosome is a ribonucleoprotein (RNA + protein) complex that catalyses the removal of introns in the processing of pre-mRNA to a mature mRNA transcript (known as splicing). It is a key tool in the post-transcriptional modification of mRNA.

A small nuclear ribonucleoprotein (snRNP) particle binds to the conserved GU sequence at the 5' end of the intron and a different snRNP binds to the conserved AG sequence at the 3' end of the intron. Upstream (towards the 5' end) of the AG consensus is a run of C and U bases called a polypyrimidine tract, and further upstream of that is the 'branch point' which includes an adenine nucleotide. All of these features are necessary for recognition by the snRNPs which together form the spliceosome complex. The multiple snRNPs come together and cause the intron to form a loop on itself, eventually being ready for excision by the now-formed spliceosome. Once excised, the introns are degraded by various exonucleases present in the nucleus.