Unrooted phylogenetic tree. The eukaryotic aspartic protease domain, PF00026 was searched against the PFAM database  and sequences under the nematode branch were retrieved. Sequences were aligned using MUSCLE  and highly similar sequences were removed from the alignment. An unrooted neighbor-joining tree was calculated from protein distances using the MEGA package [40, 41]. Gapped regions were excluded in a complete fashion and percentage bootstrap values from 500 replicates were derived. Numbers above branches indicate the relative frequency in which bipartitions were observed (only values above 50% are shown). Sequences used in the final alignment are labelled according to their accession number at UNIPROT. Organisms are indicated after the accession numbers, as followed: Ac, Ancylostoma caninum; Ace, Ancylostoma ceylanicum; Bm, Brugia malayi; Cb, Caenorhabditis briggsae; Ce, Caenorhabditis elegans; Hc, Haemonchus contortus; Mi, Meloidogyne incognita; Na; Necator americanus; Ov, Onchocerca volvulus; Sr, Strongyloides ratti; Ss, Strongyloides stercoralis. Major groups I-III referred to in the text and coloured differently. All the sequences which present the Cys-rich insertion are labelled with a star.