In our study, pathogenic variants in PRF1 were identified in two patients only and both were compound heterozygotes and carried the same deletion of five nucleotides in position 808, which has been reported in two different large cohort studies concerning the molecular background of mitochondrial diseases and breast cancer but have hitherto not been associated with HLH (Maxwell et al. 2016; Pronicka et al. 2016). The gene discussed is PRF1; the disease is mitochondrial disease.