Congenital neutropenia patients with ELA2 mutations (ELA2-CN) and cyclic neutropenia patients harbouring ELA2 mutations (ELA2-CyN) represent approx. 60% of inherited neutropenia patients in Europe and North America. Other genetic defects causing CN like HAX1-, G6PC3-mutations have been identified in minor cohorts of consanguineous families in Europe only. Although both, the phenotype of CN and CyN are related to identical ELA2-mutations, the clinical course differs significantly: ELA2-CN patients have a high risk for secondary leukemias, require higher G-CSF doses and develop G-CSF receptor mutations, whereas ELA2-CyN respond to lower G-CSF doses without malignant transformation showing the typical cycling of neutrophil counts throughout their life. It is still unknown how ELA2 mutations contribute to these different clinical phenotypes. We hypothesize the existence of modifying molecular defects and mutator genes in patients with ELA2 mutations discriminating between the two major phenotypes (CN and CyN). Within the applied network we will identify families with patients suffering from ELA2-CN or ELA2-CyN to compare the molecular findings with healthy family members by SNP technology. In addition, we will share the material to investigate the pathomechanisms of specific mutations regarding G-CSF receptor signalling, transcription factor dysfunctions, chromatin remodelling and mouse models for leukemogenesis.