| Feature | HELIX |
| Definition | Secondary structure: Helices, for example, Alpha-helix; 3 helix; or Pi-helix |
| Optional Qualifiers | /NOTE="free text" |
| Comment | This feature is used only for proteins whose tertiary structure is known. Only three types of secondary structure are specified: helices (key HELIX), beta-strands (key STRAND) and turns (key TURN). Residues not specified in one of these classes are in a "loop" or "random-coil" structure. |