To provide a method of identifying a CFTR-binding compound for treating cells having a reduced apical Cl- conductance, such as cystic fibrosis cells, and further to provide a therapeutic method of CF cells by bringing the compound in an amount effective for the therapy, selected by the identifying method into contact with the cells having the reduced apical Cl- conductance, a new compound useful for carrying out the method, and a pharmaceutical composition containing the compound.

This compound is a polypeptide Iα constituting a part of a CFTR protein. The CFTR-binding compound is used for curing the cells having the reduced apical Cl- conductance such as the cystic fibrosis cells. Especially preferably, the compound has nearly no or entirely no affinity with the adenosine cell acceptor.