Why classify proteins?

Proteins can be classified into groups according to sequence or structural similarity. These groups often contain well characterised proteins whose function is known. Thus, when a novel protein is identified, its functional properties can be proposed based on the group to which it is predicted to belong. 

In this tutorial we will explain how families, domains and sequence features can be defined and used for protein classification. Although these terms are widely used in the biological literature, you will find that their definitions may vary depending on the source. Taking this into account, lets see how  proteins can be classified into different groups based on: