Description of the Major CATH Classes
| Cath class | Description |
| Class 1 | all alpha helical protein domains |
| Class 2 | all beta sheet protein domains |
| Class 3 | alpha and beta protein domains |
| Class 4 | domain with few secondary structure elements |
| Class 5 | full PDB chains grouped into sequence families whose individual domains are classified within CATH classes 1–4 |
| Class 6 | single domain PDB chains that have not been classified within CATH classes 1–4 |
| Class 7 | full PDB chains grouped into sequence families whose individual domains are not classified within CATH classes 1–4 |
[i] Classes 1–5 represent protein structures that have been fully classified within the CATH database. Classes 6 and 7 represent proteins that have been integrated into the database but have only completed the initial classification steps.