In eukaryotes, three different "S1" proteins are bound to the ring, whereas in archaea either one or two different "S1" proteins can be part of the exosome (although there are always three S1 subunits attached to the complex).
En eucariotas únense tres proteínas S1 diferentes ao anel, entanto que en arqueas poden formar parte do exosoma un ou dous tipos destas (pero sempre hai tres subunidades S1 unidas ao complexo).