These defects comprise severe congenital neutropenia (SCN), cyclic neutropenia, CGD, leukocyte adhesion deficiencies (LADs), myeloperoxidase (MPO) deficiency, warts, hypogammablogulinaemia, infections, myelokathexix (WHIM) syndrome, specific granule deficiency, defects in toll-like receptor (TLR)/interleukin 1 receptor-associated kinase (IRAK) 4/caspase activation and recruitment domain-containing protein (CARD) 9 signalling, and other less defined entities and are summarized in the chapters below. The gene discussed is MPO; the disease is severe congenital neutropenia.