This Phase 1/2, first-in-human study will evaluate the safety and tolerability of single and multiple escalating doses of MRT5005 administered by nebulization to the respiratory tract of adult subjects with CF. This study will also assess the delivery of the drug to bronchial epithelial cells following multiple doses of MRT5005, and will characterize its biological activity by measuring changes in CFTR protein levels and CFTR chloride channel activity post-treatment.