Figure 7.

The blue arrows show the predicted evolutionary paths of β subunits from the ancestral β4 subunit. Pairings of different α and β subunits are shown by thin black lines. This concept shows two lines of evolution diverging from β1, one towards blood cell integrins consisting of β2 or β7 subunits and the other towards a cluster consisting of β5, β6, β8 and β3 subunits that are primarily expressed in tissues other than blood with exception of α IIb.β3 that is expressed also in blood platelets (see text).