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CB : current biology : dispatches from the front lines of biology
X-cells have long been associated with tumor-like formations (xenomas) in marine fish, including many of commercial interest. The name was first used to refer to the large polygonal cells that were found in epidermal xenomas from flatfish from the Pacific Northwest . Similar looking cells from pseudobranchial xenomas had previously been reported from cod in the Atlantic  and Pacific Oceans . X-cell pathologies have been reported from five teleost orders: Pleuronectiformes (flatfish), Perciformes (perch-like fish), Gadiformes (cods), Siluriformes (catfish), and Salmoniformes (salmonids). Various explanations have been elicited for their etiology, including being adenomas or adenocarcinomas [4, 5], virally transformed fish cells [6-8], or products of coastal pollution [9, 10]. It was hypothesized that X-cells were protozoan parasites [1, 11-13], and although recent molecular analyses have confirmed this, they have failed to place them in any phylum [14-18], demonstrating weak phylogenetic associations with the haplosporidians  or the alveolates . Here, we sequenced rRNA genes from European and Japanese fish that are known to develop X-cell xenomas. We also generated a metagenomic sequence library from X-cell xenomas of blue whiting and Atlantic cod and assembled 63 X-cell protein-coding genes for a eukaryote-wide phylogenomic analysis. We show that X-cells group in two highly divergent clades, robustly sister to the bivalve parasite Perkinsus. We formally describe these as Gadixcellia and Xcellia and provide a phylogenetic context to catalyze future research. We also screened Atlantic cod populations for xenomas and residual pathologies and show that X-cell infections are more prevalent and widespread than previously known.