Effect of upstream ORFs on TPO mRNA translation. (Top) Simplified drawing of TPO mRNA with only one uORF in the 5′-UTR followed by the TPO coding region (open boxes). According to the ribosomal scanning model, the 40 S ribosomal subunit will bind the cap structure at the 5′ end of the mRNA and scan the mRNA until it encounters the first AUG, where a functional ribosome is assembled. The ribosome will initiate translation and synthesize a short peptide until a stop codon is reached. Here the ribosome dissociates from the mRNA. This will prevent the ribosome from reaching the physiological TPO start codon. However, a minor proportion of 40 S subunits may remain associated with the mRNA and continue scanning for a downstream AUG. (Bottom) TPO 5′-UTR with point mutations in all uAUG codons will allow the 40 S subunit to reach the physiological start site more efficiently and initiate translation of the TPO protein.