CFTR and cystic fibrosis: However, theoretically correcting a stem cell population within the airways may provide a renewable and long-term source of endogenous cells capable of renewing the damaged epithelia with cells that express wild-type CFTR. Yet surprisingly, with the exception of a Phase I and II clinical trial for MRT5005 [https://www.cff.org/Trials/Pipeline/details/10157/MRT5005], a drug that delivers CFTR-encoded mRNA to the lungs (RESTORE-CF), there are no other clinical trials for CF gene therapy.