Assess for genetic underpinnings of primary HLH in consultation with Oncology:- Perforin (by FC*)- SAP (if male, by FC)- XIAP (male and some female, by FC)- CD107a-degranulation (FC)/NK Cytotoxicity- Hair analysis (pigment abnormalities)- Targeted genetic panel or WESEvaluation for IEI in consultation with Immunology/Genetics:- Lymphocyte subsets- T cell phenotyping- IgG, IgA, IgM titers- Vaccine titers- Neutrophil Oxidative Burst (DHR)- Targeted genetic panel or WES- Metabolic workup if indicatedPreparation for HCT:- HLA typing and initiation of donor search. Here, CD40LG is linked to hemophagocytic syndrome.