Although apparently functionally unrelated, intracellular TRAFs and extracellular meprins share a region with conserved meprin and traf homology, MATH(1). Both TRAFs and meprins require subunit assembly for function. By structural analysis of the sequences, we provide an explanation of how meprins, which form tetramers, and TRAF molecules, which form trimers, can share homology. Our analysis suggests it is highly likely that the same oligomerization surface is used. The analysis has implications for the widely distributed group of proteins containing MATH domains.