The core of the complex has a ring structure consisting of six proteins that all belong to the same class of RNases, the RNase PH-like proteins. In archaea there are two different PH-like proteins (called Rrp41 and Rrp42), each present three times in an alternating order. Eukaryotic exosome complexes have six different proteins that form the ring structure. Of these six eukaryotic proteins, three resemble the archaeal Rrp41 protein and the other three proteins are more similar to the archaeal Rrp42 protein.