RNA editing. The apo B gene, which encodes apolipoprotein B, is transcribed in the liver normally and the mRNA is translated into a 100-kDa protein (apoB100). In the intestine, however, the apo B mRNA is edited by the enzyme cysteine deaminase, which alters a CM codon (encoding a glutamine) to UM (a stop codon). This change results in synthesis of a 48-kDa form of apolipoprotein B (apoB48). The apoB100 and apoB48 proteins serve different functions in the liver and the intestine. Although RNA editing is not widespread, there are a growing number of other examples. Alternative RNA splicing is a much more common means of generating alternative forms of a protein. In splicing, intron sequences are removed and exon (protein coding) sequences are joined together. Use of alternative splice sites can include or exclude ceain exons, resulting in different forms of a protein.