Phylogenetic tree based on the nucleotide sequences of the open reading frames of the cysteine protease genes in poultry red mites (PRMs, Dermanyssus gallinae), arthropods, including other mites and ticks, chickens, and other species such as insects and invertebrates. The tree was constructed using the maximum-likelihood method with MEGA X software [14]. The numbers on the right indicate the clusters. Cluster 1: This cluster was divided into two sub-clusters. The digestive cysteine proteases from various species were classified into cluster 1–1, and cysteine proteases from PRMs (Deg-CPR-1) belonged to this cluster (bold font). Sub-cluster 1–2 included cathepsin L-like proteases and some cysteine proteases that are proposed to play a role in haemoglobin digestion in ticks (underlined). Cluster 2: This cluster comprised the cysteine proteases and cathepsins S, H, K, and L of chickens. Cluster 3: The cysteine proteases from mites, different from those in cluster 1 were included in this cluster, and another cysteine protease from PRMs, Dg-CatL-1, was also included (bold font).