A protein family is a group of evolutionarily-related proteins, and is often nearly synonymous with gene family. 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 (see homology) and typically have similar three-dimensional structures, functions, and significant sequence similarity. While it is difficult to evaluate the significance of functional or structural similarity, there is a fairly well developed framework for evaluating the significance of similarity between a group of sequences using sequence alignment methods. Proteins that do not share a common ancestor are very unlikely to show statistically significant sequence similarity, making sequence alignment a powerful tool for identifying the members of protein families.
Currently, over 60,000 protein families have been defined, although ambiguity in the definition of protein family leads different researchers to wildly varying numbers.
Read more about Protein Family: Terminology and Usage, Protein Domains and Motifs, Evolution of Protein Families, Use and Importance of Protein Families, Protein Family Resources
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“Every family has one passage of scripture they stumble over.”
—Chinese proverb.