Relative alignments among TM1, TM6, and TM12 depicted in a CFTR homology model. Based on the crystal structure of an asymmetrical ABC transporter, TM287/288, Corradi et al. (2015) derived a CFTR model in which the alignment between TM1 and TM6 (left) is fairly consistent with data represented in the current work. However, the distance between several positions where introduced cysteines were shown to be cross-linkable (El Hiani and Linsdell, 2014) is way beyond the physical constrain for the formation of an effective Cd²⁺ metal bridge. For example, the distance between M348 and S1141 (15.5 Å, middle) and that between K95 and S1141 (17.6 Å, right) both well exceed the working distance of Cd²⁺, the cross-linker used in El Hiani and Linsdell (2014). Note that although the distances measured above are between α-carbons of the two residues involved, the minimum sulfur to sulfur distance between two engineered cysteines, calculated by subtracting 5.6 Å (twice the distance of a single cysteine side chain) from the above values (i.e., 9.9 Å for 348C and 1141C; 12 Å for 95C and 1141C), is still >6.5 Å, the distance for coordination of two thiol groups with Cd²⁺.