A protein that terminates translation of messenger RNA (mRNA) during protein synthesis and releases the completed polypeptide chain from the ribosome. It recognizes a stop codon in the mRNA and attaches to the A site of the ribosome (the binding site for an aminoacyl tRNA molecule), blocking further elongation of the polypeptide chain. Then the mRNA is released and the ribosome complex dissociates, releasing the polypeptide. Prokaryotes have two principal release factors, RF-1 and RF-2, whereas eukaryotes have just one, eRF.