The protein occurs in the plasma in 2 main isoforms, ApoB48 and ApoB100. The first is synthesized exclusively by the small intestine, the second by the liver. ApoB-100 is the largest of the apoB group of proteins, consisting of 4563 amino acids. Both isoforms are coded by "APOB" and by a single mRNA transcript larger than 16 kb. ApoB48 is generated when a stop codon (UAA) at residue 2153 is created by RNA editing. There appears to be a "trans"-acting tissue-specific splicing gene that determines which isoform is ultimately produced. Alternatively, there is some evidence that a "cis"-acting element several thousand bp upstream determines which isoform is produced.