Table 3.

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

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.