In five (four with active HLH and one under treatment) of the eight FHL-4 samples, we detected higher frequencies of naive (IgD+CD27+) B cells compared with the age-related reference values (Piatosa et al., 2010), whereas only one out of 11 FHL-2/3 patients had elevated levels of naive B cells (Fig. 1 B, upper left). This evidence concerns the gene CD27 and hemophagocytic syndrome.