Neighbor-joining phylogenetic tree of Argonaute/PIWI protein family. Major protein subgroups are labeled. The tree is unrooted. Phylogenetic reconstruction method, species colour-coding and nodes labeling of significance are the same as Fig. 1. Human and Drososphila PIWI/Ago proteins discussed in the text are in larger font. The branch leading to a putative, but unlikely, fifth Argonaute gene homolog in mouse, mAgo5, is labeled with a large arrow (see text for explanation). Other branches are labeled by a four letter species identifier (the first two letters from the genus and species names) and the GenBank accession number). Species name abbreviations are given in the Methods. The multiple sequence alignment file is given in Additional file 2.