What's release factor (RF) and Nonsense-mediated decay (NMD) ?

Release factor (RF) – A protein required to terminate polypeptide translation to cause release of the completed polypeptide chain and the ribosome from mRNA. Nonsense-mediated decay (NMD) targets mRNAs with premature stop codons. Targeting of NMD substrates requires a conserved set of Upf and SMG proteins.