This is the highest level of classification and is derived from the gross secondary structure content. There are 4 classes defined in the CATH classification.

1. Mainly Alpha
2. Mainly Beta
3. Alpha Beta
4. Few Secondary Structures.

The classification of protein domains into these 4 classes has been achieved for 90% of structures automatically, using an automated class assignment protocol. This classification protocol includes a number of criteria:

• Percentage composition of secondary structures
• Percentage of secondary structures contacts
• Secondary structure alternation
• Percentage of parallel strands

This is discussed in detail in the paper referenced below.