Focus issue: Evolution III--domains for change

This third installment in the series on the evolution of signaling molecules and networks highlights domains as a medium for evolution. Two Perspectives highlight how changes in domains can lead to new protein interactions or the evolution of new signaling events. Research in this issue highlights the evolution of proteins with domains involved in extrinsic apoptosis, a particular form of cell death caused by extracellular ligands that bind to "death receptors," and shows that this cell death pathway occurred earlier in evolution than was previously thought. A Review describes the evolution of the ABL family of tyrosine kinases, which have acquired isoform-specific functions through the unique combinations of domains and which are of particular importance in cancer.