What is removed during mRNA processing?

Introns are removed during mRNA processing. This is an essential step for proper protein expression. 

The DNA of eukaryotes includes exons, which code for genes, and introns, which are non-coding regions. If introns are retained, the RNA would be translated into a nonfunctional protein. To prevent this from happening, introns are removed from the pre-mRNA by a large protein complex known as the spliceosome. The exons, which are retained, are spliced back together in the nucleus before the RNA migrates to the cytoplasm. When splicing is complete, the mature mRNA containing uninterrupted coding information is transported to the cytoplasm where it is translated into proteins by ribosomes.