Fig. 1. Examples of different types of protein–protein interactions as described in the text: (A) obligate homodimer, P22 Arc repressor; (B) obligate heterodimer, human cathepsin D that consists of a non-homologous light (red) and heavy (green) chain; (C) non-obligate homodimer, sperm lysin; (D) non-obligate heterodimer, RhoA (green) and RhoGAP (red) signalling complex; (E) non-obligate permanent heterodimer, thrombin (red) and rodniin inhibitor (green); (F) non-obligate transient heterotrimer, bovine G protein, i.e. the interaction between Gα (green) and Gβγ (red, orange) is transient.