Acknowledgements
The authors thank the patients with cystic fibrosis for their participation in this study; A. Fichtner and J. Pfeil for performing rectal biopsy procedures; C. Rueckes-Nilges, C. Herth, S. Tamm, C. Berger, and S. Engelhardt for excellent technical assistance; and S. Barth, S. Hämmerling, S. Junge, A. Sauer-Heilborn, F. Ringshausen, O. Sommerburg, and J. Schäfer-Feterowski for clinical care of study participants