During evolution genes can produce more complex proteins by gene fusion or less 
complex proteins by gene fission. Considering proteins from 131 completely 
sequenced genomes from all three kingdoms of life, we identified 2869 groups of 
multi-domain proteins as a single protein in certain organisms and as two or 
more smaller proteins with equivalent domain architectures in other organisms. 
We found that fusion events are approximately four times more common than 
fission events, and we established that, in most cases, any particular fusion or 
fission event only occurred once during the course of evolution.