NHD is a very rare autosomal recessive disease, and the responsible two genes, which are expressed in microglia in the brain, are DNAX-activation protein of 12 kDA (DAP12) and triggering receptor expressed on myeloid cells 2 (TREM2; Paloneva et al., 2000, 2002). Here, TREM2 is linked to Nasu-Hakola disease.