* F: Formylation (optional)
* A: Adenylation (required in a module)
* PCP: Thiolation and peptide carrier protein with attached 4'-phospho-pantetheine (required in a module)
* C: Condensation forming the amide bond (required in a module)
* Cy: Cyclization into thiazoline or oxazolines (optional)
* Ox: Oxidation of thiazolines or oxazolines to thiazoles or oxazoles (optional)
* Red: Reduction of thiazolines or oxazolines to thiazolidines or oxazolidines (optional)
* E: Epimerization into D-amino acids (optional)
* NMT: N-methylation (optional)
* TE: Termination by a thio-esterase (only found once in a NRPS)
* R: Reduction to terminal aldehyde or alcohol (optional)
* X: Recruits cytochrome P450 enzymes (optional)