In a recent report by Abdalgani et al. (11), where accuracy of perforin expression by flow cytometry was evaluated, it was concluded that, compared to patients with biallelicPRF1 mutations, patients with monoallelic mutations, variants of uncertain clinical significance, and a minority of HLH patients without PRF1 mutations, had normal perforin expression but lower mean fluorescence intensity (MFI) (11). The gene discussed is PRF1; the disease is hemophagocytic syndrome.