In addition, the lack of acute efficacy of CFTR blockers (CFTRinh172 and GLYH101, Figure S2 in File S1) and inconsistent increases in sodium absorption in CF biopsies (Figure 2, Table 2, Figure S1 in File S1) could be limitations to use of ICM as a CFTR biomarker in modulator trials. This evidence concerns the gene CFTR and cystic fibrosis.