Protein family

A protein family is a group of evolutionarily related proteins. In many cases, a protein family has a corresponding gene family, in which each gene encodes a corresponding protein with a 1:1 relationship. The term "protein family" should not be confused with family as it is used in taxonomy.

Proteins in a family descend from a common ancestor and typically have similar three-dimensional structures, functions, and significant sequence similarity.^[1][2] Sequence similarity (usually amino-acid sequence) is one of the most common indicators of homology, or common evolutionary ancestry.^[3][4] Some frameworks for evaluating the significance of similarity between sequences use sequence alignment methods. Proteins that do not share a common ancestor are unlikely to show statistically significant sequence similarity, making sequence alignment a powerful tool for identifying the members of protein families.^[3][4] Families are sometimes grouped together into larger clades called superfamilies based on structural similarity, even if there is no identifiable sequence homology.

Currently, over 60,000 protein families have been defined,^[5] although ambiguity in the definition of "protein family" leads different researchers to highly varying numbers.