The noncoding sequences in the UTR between the Gene Start signal (GS) and the initial codon AUG from 25 measles virus genomes were compared. The conserved sequences 5'-U(NNNNN)(5-8) -3' and 5'(CNNNNNC)-3' were found in the genome and the UTR of phosphoprotein (P) and Fusion (F) mRNA. These sequences were named the U-Unit and the C-Unit, respectively. These two sequences are different in the P and F mRNAs. To investigate whether these conserved sequences can be used to regulate expression of foreign proteins in additional transcription units (ATU), a series of minigenomes with various 5' UTRs were generated. From reporter gene assays and quantitation of gene copies by qPCR, we found that the introduction of the C-unit into the 5'UTR of mRNA can down-regulate the expression of foreign proteins in additional transcription units in vivo. (C) 2018 The Authors. Published by Elsevier Inc.