| Feature | SOURCE |
| Definition | Identifies the biological source of the specified span of the sequence; this key is mandatory; more than one source key per sequence is allowed; every entry/record will have, as minimum, either a single source key spanning the entire sequence or multiple source keys, which together, span the entire sequence |
| Mandatory Qualifiers |
/MOL_TYPE="protein" /ORGANISM="free text" |
| Optional Qualifiers | /NOTE="free text" |