selection of the edited cytidine is also highly dependent on this secondary structure of the surrounding rna. there are also some indications that this loop region is formed between the mooring sequence and the 3′ regulatory region of the apob mrna. the predicted secondary structure formed by apob mrna is thought to allow for contact between the residue to be edited and the active site of apobec1 as well as for binding of acf and other auxiliary factors associated with the editosome. regulation editing of apob mrna in humans is tissue regulated, with apob48 being the main apob protein of the small intestine in humans. it occurs in lesser amounts in the colon, kidney and stomach along with the non edited version. editing is also developmentally regulated with the non edited version only being translated early in development but the edited form increases during development in the tissues where editing can occur. editing levels of apob mrna have been shown to vary in response to changes in diet. exposure to alcohol and hormone levels. conservation apob mrna editing also occurs in mice, and rats. in contrast to humans editing occurs in liver in mice and rats up to a frequency of 65%. it has not been observed in birds or lesser species. structure editing results in a codon change creating an in-frame stop codon leading to translation of a truncated protein, apob48. this stop codon results in the translation of a protein that lacks the carboxyl terminus which contains the protein's ldlr binding domain. the full protein apob100 which has nearly 4500 amino acids is present in vldl and ldl. since many parts of apob100 are in an amphipathic condition, the structure of some of its domains is dependent on underlying lipid conditions. however, it is known to have the same overall folding in ldl having five main domains. recently the first structure of ldl at human body temperature in native condition has been found using cryo-electron microscopy at a resolution of 16 angstrom. the overall folding of apob-100 has been confirmed and some heterogeneity in the local structure of its domains have been mapped. function editing is restricted to those transcripts expressed in the small intestine. this shorter version of the protein has a function specific to the small intestine. the main function of the full length liver expressed apob100 is as a ligand for activation of the ldl-r. however, editing results in a protein lacking this ldl-r binding region of the protein. this alters the function of the protein and the shorter apob48 protein as specific functions relative to the small intestine. apob48 is identical to the amino-terminal 48% of apob100. the function of this isoform is in fat absorption of the small intestine and is involved in the synthesis, assembly and secretion of chylomicrons. these chylomicrons transport dietary lipids to tissues while the remaining chylomicrons along with associated residual lipids are in 2–3 hours taken up by the liver via the interaction of apolipoprotein e (apoe) with lipoprotein receptors. it is the dominant apob protein in the small intestine of most mammals. it is a key protein in the exogenous pathway of lipoprotein metabolism. intestinal proteins containing apob48 are metabolized to chylomicron remnant particles which are taken up by remnant receptors.