Polypyrimidine tract

The polypyrimidine tract is a region of pre-messenger RNA (mRNA) that promotes the assembly of the spliceosome, the protein complex specialized for carrying out RNA splicing during the process of post-transcriptional modification. The region is rich with pyrimidine nucleotides, especially uracil, and is usually 15–20 base pairs long, located about 5–40 base pairs before the 3' end of the intron to be spliced.