The annelid Nereis virens had detectable aromatase activity, likely occurring in the gut epithelium. Despite having detectable aromatase activity, the SCH772984 942183-80-4 protein responsible for this function remains unknown. CYPome studies in annelid species may provide clues to the evolution of the steroidogenesis pathway in metazoa and whether annelid invertebrates utilize the same enzymes for de novo sex steroid production. All of the C. telata clan 2 CYPs were located in novel CYP families; indeed the C. telata clan 2 sequences had 14 novel CYP families made up from 20 subfamilies. It has been postulated that a large number of CYPs related to families involved in exogenous metabolism may suggest evolutionary pressure towards diverse function. The largest family was CYP3052 with 24 sequences; these sequences made up the majority of the large standalone cluster of 33 C. teleta CYPs on the phylogenetic tree. If this family of C. teleta CYPs follows the trend of other large CYP families, namely families CYP1–4, then these proteins may be involved in xenobiotic metabolism. There were five novel families with a single sequence each that were CYP1-like. There were fewer CYP1-like genes in C. telata than were found in S. purpuratus but similar to what is typical in vertebrates. Of the two families that grouped with CYP2s, CYP3058 clustered more closely with C. elegans sequences than vertebrate CYP2 sequences in the clan 2 phylogeny. The other family, CYP3059, clustered with vertebrate CYP2R, the placement of this family is uncertain with respect to the vertebrate CYP2 families. The function of the C. elegans CYPs are unknown but vertebrate CYP2s are well known for their role in xenobiotic metabolism. CYP3057A1 and CYP3064A1 were basal in this clan and had high divergence from the remaining sequences. The clan 3 phylogeny had sequences from across all metazoan phyla. Clan 3 contains families CYP3 and CYP5 in vertebrates, but is represented by different families in invertebrates such as families CYP6 and CYP9. Mammalian CYP3s are known to have very flexible active sites that can accommodate structurally diverse substrates. CYP3A4 is the most important enzyme involved in drug metabolism in humans but other CYP3s are also important in metabolism of endogenous and exogenous compounds. Clan 3 CYPs are involved in both endogenous and exogenous metabolism in arthropods. C. teleta had two clan 3 families with a total of nine CYPs; both families were novel. N. vectensis had 20 clan 3 CYPs, S. purpuratus had 10, and D. melanogaster has an expanded clan 3 with 36 CYPs. Mammals appear to have a much smaller number of clan 3 genes than many invertebrate species; humans have just five clan 3 sequences from a single subfamily. The C. teleta clan 3 sequences included CYP331A1, which had been previously described. CYP331A1 had increased expression from exposure to benzopyrene and fluoranthene, two PAHs. The CYP331 family has been expanded in this annotation with two more CYP331A genes and the CYP331B1 gene.