The specificity of the assay for CFTR activity is tested using two different CFTR inhibitors, CFTR-172 and PPQ-102, and downregulating CFTR (siRNA technology) in the acute monocytic leukaemia MM6 cell line, highly expressing CFTR. The outcome was confirmed also in peripheral blood leukocytes collected by venipuncture (3-5 mL), performed in different centers and by different operators. Significant differences between WT and CF peripheral blood mononuclear cells (PBMC) and purified monocytes were recorded. Of special interest were the results obtained by the HS-YFP assay performed in PBMCs of a patient carrying the G1349D/F508del mutation and taking Ivacaftor; and the results in 18 patients in a PTC clinical trial, in 7 patients taking Orkambi, in 4 patients in clinical trial with VX 770+VX 661. 14 CF patients homozygotes for S1251N mutation and treated with ivacaftor were studied (by Rotterdam researchers) and HS-YFP assay seemed to confirm the positive effect of ivacaftor. Even if several assay variables that need further optimization among partners of research project were identified, HS-YFP assay might represent a new convenient method to measure CFTR function and the effect of correctors and potentiators on defective CFTR function. It represents a further step toward a personalized medicine approach in CF.