Familial Hemophagocytic Lymphohistiocytosis (FHL) is a rare immunodeficiency causing insufficient defence of the child or infant from infections, through defective killing of virus-infected and stimulating dendritic cells. The clinical picture resembles leukemia; final diagnosis comes from specific immune tests and mutation analysis. Among FHL-related genes recognized since 1999, UNC13D is responsible for a wide proportion of cases (FHL-3), up to 40% in some European countries. Accumulation of sufficient cases allows to understand different clinical manifestations of FHL and correlate them to different mutations. This may have implications for identification of patients with atypical pictures or late onset.   (By Dr. Maurizio Aricò, http://jmg.bmj.com/content/early/2011/01/19/jmg.2010.085456 )