Familial hemophagocytic lymphohistiocytosis is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Five genetic subtypes of the familial form of the disease (FHL1, FHL2, FHL3, FHL4, and FHL5) are described.

The five subtypes of FHL are each associated with a specific gene:

• FHL1 – HPLH1
• FHL2 – PRF1 (Perforin)
• FHL3 – UNC13D (Munc13-4)
• FHL4 – STX11 (Syntaxin 11)
• FHL5 – STXBP2 (Syntaxin binding protein 2)/UNC18-2

Molecular genetic testing for four of the causative genes, PRF1 (FHL2), UNC13D (FHL3), STX11 (FHL4), and STXBP2 (FHL5), is available on a clinical basis.