Evaluation of NK/CTL GrB detection in patients with macrophage activation syndrome (MAS) in patients with juvenile idiopathic arthritis at our center revealed variable results, with increased detection in some, but not all patients with active disease (Grom et al., 2003) and data not shown), highlighting the subtle differences between HLH and MAS. The gene discussed is GZMB; the disease is hemophagocytic syndrome.