[Sucecesfull bone marrow transplantation in a case of familial hemophagocytic lymphohistiocytosis type 3].

Hemophagocytic lymphohistiocytosis (HLH) is an exaggerated activation of the immune system which can be either primary (familial) or secondary. Familial hemophagocytic lymphohistiocytosis type 3 (FHL-3) is a severe immune disorder, caused by mutations in the UNC13D gene, which codes for a protein crucial to the cytotoxic function of lymphocytes. To describe the diagnostic relevance of next-generation sequencing in the approach of a patient with suspected FHL and to demonstrate the effectiveness of bone marrow transplantation as the only curative measure. 4-year-old preschool male, previously healthy, who presented with mononucleosis syndrome and positive IgM for Epstein Barr virus, developing hepatosplenomegaly and progressive clinical de terioration. A lymphoproliferative syndrome was suspected, which was ruled out by bone marrow aspiration, finding evidence of active hemophagocytosis. The patient met the criteria for hemophago cytic syndrome (bone marrow aspiration, pancytopenia, elevated ferritin, and hypertriglyceridemia) and, given the lack of response to first-line management, including antiviral treatment, a possible primary etiology was considered. A molecular study was completed with NGS that was positive for FHL-3. Due to the progressive clinical deterioration, a bone marrow transplantation was performed, presenting successful results after the first year had elapsed. NGS is an indispensable tool in the diagnosis of FHL, mainly when the response to standard treatment is not adequate and facilitates the timely implementation of the necessary therapeutic measures.