Plotting of VI/F+I/F/C values for the three groups (classical CF, atypical CF, and non-CF) evidences a clear distinction among these groups (Figure 2), where more negative values of VI/F+I/F/C are associated with non-CF controls, intermediate values (i.e., residual CFTR activity) to PwCF with atypical disease, and higher VI/F+I/F/C values (i.e., absence of functional CFTR) correspond to PwCF with classical CF forms. Here, CFTR is linked to cystic fibrosis.