CFTR and cystic fibrosis: The deletion of a phenylalanine at position 508 (p.Phe508del, legacy: F508del) is the most prevalent CF-causing variant, accounting for approximately 70% of all CF alleles [1], while the remaining 30% of CF alleles are represented by an enormous number of CFTR variants and most are (ultra)rare, occurring among few PwCF worldwide [11].