Firstly, a branch with sequences from very diverse organisms that can be loosely labeled together as marine picoeukaryotes. Secondly, the tree includes a branch with metazoan sequences, homologues of vertebrate SELO2 and sequence from other taxons. The third branch contains plant and green algal proteins. The fourth one is mostly composed of fungal sequences. Separate in the super-branch is the sequence from the excavate Naegleria. Notable is the absence of the SELO gene in all nematodes Veratramine including C. elegans and in most arthropods, including Drosophila and all insects. Thus, SELO is probably a pan-eukaryotic gene that has been lost from a number of genomes. A phylogenetic tree including representative sequences from all domains of life as well as the marine metagenomic sequences, unassigned to a taxon, shows a similar picture, albeit highlighting the difficulties in elucidating the evolutionary relationships between eukaryotic and prokaryotic SELO homologues.Can the kinase function prediction for SELO proteins be trusted, or is it only a reliable three-dimensional fold prediction? Nevertheless, the most plausible evolutionary scenario would involve SELO as an ancient gene in the last universal ancestor of bacteria. Accordingly, SELO remains a mitochondrial protein in all the studied eukaryotes. SELO gene loss in several eukaryotic and bacterial lineages would have produced the taxonomic spread observed today. Secondary structure predictions and sequence alignments to the known kinase-like structures according to structure prediction results suggest that the SELO domains are composed of an N terminal smaller lobe, mostly composed of b strands, and a predominantly helical, larger, C-terminal lobe. These secondary structure elements form the core of the two lobes of a typical kinase-like fold protein. These predictions are in agreement with the kinase domain secondary structure topology. Within the SELO alignment, there are some conserved insertions, e.g. between SELO and SELO2 Magnoflorine-iodide groups, and in the fungal sequences, as compared to the other SELO proteins. These usually occur outside the predicted secondary structure elements.