The difference between enzymatically active and inactive homologues has been noted (and in some cases, understood when comparing catalytically active and inactive proteins residing in recognisable families) for some time at the sequence level, and some pseudoenzymes have also been referred to as prozymes when they were analysed in protozoan parasites. The best studied pseudoenzymes reside amongst various key signalling superfamilies of enzymes, such as the proteases, the protein kinases, protein phosphatases and ubiquitin modifying enzymes. The role of pseudoenzymes as "pseudo scaffolds" has also been recognised  and pseudoenzymes are now beginning to be more thoroughly studied in terms of their biology and function, in large part because they are also interesting potential targets (or anti-targets) for drug design in the context of intracellular cellular signalling complexes.