UCART123 (referred to as the “study product” or “UCART123 cells”) is a new, investigational form of leukemia therapy in which healthy immune system cells, called T cells, are taken from healthy donors (these are called allogeneic T cells) and specially engineered in the laboratory to be able to target a molecule on the surface of leukemia cells called CD123. Preliminary data from animal studies have suggested that this process can generate allogeneic T cells that could be highly active against AML cells that express CD123. If the study product is given to a patient with AML, it is hoped that the T cells will be able to destroy the CD123 positive leukemia cells and lead to remission in selected patients with AML. In case the study product is causing side effects, the study product has been engineered to allow it to be destroyed by administering a drug called Rituximab.