Cover picture: Stereo lateral view of the transmembrane domains (TMDs) of zebrafish CFTR in an inward-facing closed state configuration (adopted from Zhang and Chen. 2016. Cell. http://dx.doi.org/10.1016/j.cell.2016.11.014). The aqueous internal vestibule (in light blue with light blue shadow) tapers down toward a narrow region close to the external end of the ion permeation pathway. Several pore-lining residues relevant to the article published in the current issue from TM5 (black), 6 (cyan), and 9 (magenta) are shown in colored balls (R303 in orange, N306 in yellow, M348 in pink, R352 in blue, and D993 in green; see Research Article by Zhang and Hwang, 355–372).