Spliceosome, molecular model. The spliceosome comprises small nuclear ribonucleoproteins (snRNPs) which perform the cleavage of pre-mRNA to excise non-coding regions called introns, and merge the remaining coding regions (exons) to form mature mRNA. This post-transcriptional process is commonly called splicing. The spliceosome ribonucleic acid (RNA) strand pairs with the pre-messenger RNA (pre-mRNA, yellow), through complementary base pairing with the sequence of its splice site (green). Following this binding, additional protein (orange, pink) and RNA subunits are recruited to assemble the rest of the spliceosome complex. This catalytic cleavage process also requires coordination from two magnesium ions. Self-splicing RNA strands can also exist which splice themselves using similar mechanisms but without the use of spliceosomes.