A disease is considered rare, when fewer than 1 in 2.000 individuals is affected. Children with rare diseases have traditionally been considered “therapeutic orphans” since the development and commercialization of therapies to treat affected patients rarely promise lucrative financial gains. In view of great progress in molecular and cellular sciences, the emergence of biotechnological tools as well as the rapid development of computer sciences, there is increasing hope for children and their families that previously incurable diseases may in fact be curable in the foreseeable future. Furthermore, disease-overarching perspectives will allow us to re-interpret phenotypes and genotypes. Rare diseases, mostly monogenic conditions, open new avenues of research to understand genes and pathways controlling differentiation, homeostasis and function of cells and organs. While there are historic and ethical reasons to distinguish rare and common diseases, this distinction should not make us believe that rare and common diseases should be fundamentally viewed as separate entities. In fact, translational sciences in rare diseases are critical to define innovative therapeutic principles for patients with common diseases. This thesis will be supported by clinical and scientific data from children with rare diseases of the blood and immune system.