CFTR and cystic fibrosis: This is the first study to investigate the effects of combination therapy with the CFTR modulators lumacaftor and ivacaftor (LUM/IVA) on CFTR function in organ systems such as sweat glands and the intestine using the CFTR biomarkers SCC and ICM in a cohort of 12 children with CF with two F508del less than 12 years of age and to correlate these results with short-term clinical outcome.