The frequency of mutations in HLH-related genes varies by country, with PRF1 and UNC13D mutations accounting for approximately 55% and 32% of Familial hemophagocytic lymphohistiocytosis (FHL) in Japan, respectively, while UNC13D mutations account for the majority of FHL in Korea [12]. The gene discussed is UNC13D; the disease is hereditary hemophagocytic lymphohistiocytosis.