CFTR and cystic fibrosis: Previous nPD and ICM studies examining patients with CF with a spectrum of CFTR variants with residual function demonstrated CFTR mediated ion transport in the range of about 10%–50% of normal and showed that inherent residual function greater than about 10% of normal may be associated with long-term exocrine pancreatic sufficiency (Hirtz et al., 2004; Wilschanski et al., 2006; Stanke et al., 2008; Derichs et al., 2010).