Fig. 13. Structure formability of hypothetical proteins composed of 4 amino acids, which are represented by one-letter symbols in brackets. They were picked up from columns and rows of the universal genetic code table. Symbols of O and X outside of brackets represent whether hypothetical proteins composed of the 4 amino acids can satisfy the 4 conditions (hydropathy, _-helix, _-sheet and _-turn formabitities) necessary for formation of globular structures or not, respectively.