Definition for 5' untranslated region
From Biology Forums Dictionary
A region of a gene which IS transcribed into mRNA, becoming the 5' end of the message, but which does not contain protein coding sequence. The 5'-untranslated region is the portion of the DNA starting from the cap site and extending to the base just before the ATG translation initiation codon. While not itself translated, this region may have sequences which alter the translation efficiency of the mRNA, or which affect the stability of the mRNA.