Control Devices are disclosed including RNA destabilizing elements (RDE), and RNA control devices, combined with transgenes, including Chimeric Antigen Receptors (CARs) in eukaryotic cells. RDEs can be combined with RNA control devices to make RDEs that include ligand mediated control. These smart RDEs and other RDEs can be used to optimize expression of transgenes, e.g., CARs, in the eukaryotic cells so that, for example, effector function is optimized. CARs and transgene payloads can also be engineered into eukaryotic cells so that the transgene payload is expressed and delivered at desired times from the eukaryotic cell.