It was also observed that the R47H variant of the triggering receptor expressed on myeloid cells 2 (TREM-2) represents an important genetic risk factor for PD [83], as well as some variants of the human leukocyte antigen gene (HLA-DRA, which codes for the HLA-DR receptor) specifically expressed on microglia [84]. Here, TREM2 is linked to Parkinson disease.