Loss of function mutations in the triggering receptor expressed on myeloid cells 2 (TREM2) gene or DNAX-activating protein of 12 kDa (DAP12) gene deletions cause polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy [also known as Nasu-Hakola disease (NHD)], a rare disease characterized by ankle pain and swellings [19, 20]. The gene discussed is TREM2; the disease is Nasu-Hakola disease.