  First Draft of Physical Characteristics of 3'UTR Length/Composition  In the mammalian genome there is considerable variation in the length of 3’-UTRs. This region of the mRNA transcript can range from 60 to 80 nucleotides to about 4000. (Hesketh) The average length for the 3’-UTR in humans is approximately 800 nucleotides meanwhile the average length of 5’UTRs is only about 200 nucleotides (Mignone). In general the longer the 3’-UTR is the more likely that lower levels of expression will be observed. This is so because longer 3’-UTRs will potentially possess more miRNA binding sites that have the ability to inhibit translation. The nucleotide composition between the 5’ and 3’-UTR also differences significantly. The mean G+C percentage of the 5’-UTR in warm-blooded vertebrates is about 60% and only 45% for 3’-UTRs. A significant inverse correlation has been observed between the G+C% of 5’ and 3’-UTRs and their corresponding lengths. Accordingly, UTRs that are GC-poor tend to be longer than those in GC-rich genome regions. (Mignone)  Degradation/stability  Sequences within the 3’-UTR also have the ability to degrade or stabilize the mRNA transcript. Modification of transcript stability allows expression to be rapidly controlled without altering translation rates. One group of the stabilization elements that are situated in the 3’-UTR are the AU-rich elements (AREs). These elements range in size from 50-150 base pairs and generally contain multiple copies of the pentanucleotide AUUUA. Early studies indicated that AREs can be variable in sequence and have defined three main classes that differ in the number and arrangement of motifs. (Barret) Depending upon the protein that binds to the region the mRNA can either be stabilized or degraded. Another set of elements that is present in both the 5’ and 3’-UTR are iron response elements (IREs). The IRE is a stem-loop structure within the untranslated regions of mRNAs that encode proteins involved in cellular iron metabolism. Whether the mRNA is degraded or stabilized is dependent on the binding of specific proteins and intracellular iron concentrations. (Hesketh)  Additions due to 3’UTR  The 3’-UTR also contains sequences that signal additions to be made either to the transcript itself or to the product of translation. There are two different polyadenylation signals present within the 3’-UTR which signal the addition of the poly(A) tail. The poly(A) tail is synthesized at a defined length of about 250 base pairs (Barret).The primary signal used is the nuclear polyadenylation signal (PAS) with the sequence AAUAAA located toward the end of the 3’-UTR. (Hesketh). In addition, cytoplasmic polyadenylation can occur which regulates the translational activation of maternal mRNAs in early development. The element that controls this process is called the CPE which is AU-rich and located near the 3’-UTR as well. The CPE generally has the structure UUUUUUAU and is usually within 100 base pairs of the nuclear PAS. (Hesketh) Another specific addition signaled by the 3’-UTR is the incorporation of selenocysteine at UGA codons of mRNAs encoding selenoproteins. Normally the UGA codon encodes for a stop of translation, but a conserved step-loop structure called the selenocysteine insertion sequence (SECIS) causes for the insertion of selenocysteine instead. (Mignone)