Protein family

[3][4] Families are sometimes grouped together into larger clades called superfamilies based on structural similarity, even if there is no identifiable sequence homology.

[2][10] Subfamilies can be defined within families to denote closely related proteins that have similar or identical functions.

Due to evolutionary shuffling, different domains in a protein have evolved independently.

A protein–protein binding interface may consist of a large surface with constraints on the hydrophobicity or polarity of the amino-acid residues.

Certain gene/protein families, especially in eukaryotes, undergo extreme expansions and contractions in the course of evolution, sometimes in concert with whole genome duplications.

[15] This expansion and contraction of protein families is one of the salient features of genome evolution, but its importance and ramifications are currently unclear.

[16] The algorithmic means for establishing protein families on a large scale are based on a notion of similarity.

The human cyclophilin family, as represented by the structures of the isomerase domains of some of its members
Phylogenetic tree of RAS superfamily: This tree was created using FigTree (free online software).