The high precision and fidelity of the genetic message transmission are ensured by numerous proofreading steps, from DNA replication and transcription to protein translation. The key event for translational fidelity is the proper codon assignment for 20 canonical amino acids. An experimental codon reassignment is possible for noncanonical amino acids in vivo using artificially constructed expression hosts under efficient selective pressure. However, such amino acids may interfere with the cellular metabolism and thus do not belong to the 'first' or 'restricted' part of the universal code, but rather to a second or 'relaxed' part, which is limited mainly by the downstream proofreading in the natural translational machinery. Correspondingly, not all possible alpha-amino acids can be introduced into proteins. The aim of this study is to discuss biological and evolutionary constraints on possible candidates for this second coding level of the universal code. Engineering of such a 'second' code is expected to have great academic as well as practical impact, ranging from protein folding studies to biomedicine.