Domain architecture of proteins containing the SPK domain. Only proteins with distinct modular organizations are shown. The domain names are those of the Simple Modular Architecture Research Tool (http://smart.embl-heidelberg.de) (Schultz et al. 1998, 2000). CASc, catalytic domain of caspases; PHD, PHD C4HC3 zinc finger; SET, (Su(var)3–9, Enhancer-of-zeste, Trithorax) domain; STYKc, catalytic domain of protein kinases. The UCH-2 (ubiquitin carboxy-terminal hydrolase family 2) domain is defined by Pfam (Bateman et al. 2000)