Kostmann’s disease is a rare autosomal recessive form of severe congenital neutropenia, characterized by the cessation of promyelocyte/myelocyte maturation in the bone marrow caused by homozygous mutations in the X1 gene encoding the mitochondrial protein HCLS1 [39], resulting in severe recurrent bacterial infections in early infancy. Here, HCLS1 is linked to severe congenital neutropenia.