ISG15 and cystic fibrosis: Moreover, as there are contrasting results on the relationship between IFN-I and ACE2 isoforms expression [13,14], we aimed to study the relationship of ACE2 (all isoforms), furin and TMPRSS2 with IFN stimulated gene 15 (ISG15), a well-established ISG that acts as a potential modifier of CF severity, as well as ACE2 responsiveness to ex vivo IFN-I stimulation [15].