Every time a hospital needs a bone marrow donor, the REDMO initiates a search that crosses registries, hospitals, and borders. At the helm is Sergi Querol, a hematologist specializing in transplants and cell therapy. Before directing the Bone Marrow Donors Registry, managed by the Josep Carreras Foundation, he spent more than twenty years leading the Barcelona Cord Blood Bank and participated in the development of the British Anthony Nolan bank. Querol explains why 44 million registered people do not guarantee finding a compatible donor, how this international network works, and why the challenge now is to incorporate more young people into it.