TLR9 and autoimmune disease: Even though CpG-DNA is scarce in mammals and mostly methylated, TLR9 can recognize microbial-specific unmethylated CpG-DNA in the human body and abnormal response targeting of self-DNA by TLR9 can trigger development of autoimmune diseases, such as psoriasis, autoimmune arthritis, and ulcerative colitis (21–24).