How the genetic code is coupled with the protein synthesis at a nanoscale distance is shrouded in mystery. Here we showthat the coupling stems froma spin current and its polarization. The codon sends a spin of a certain orientation to the tRNA end. The spin recognizes its counterpart on the amino acid. If the spins are of opposite direction, the synthesis is allowed; if not, the synthesis is forbidden. The rule is flawless and rests on quantum computational experiment.