Algorithms suggest that NCKX proteins consist of an N-terminal signal peptide and 11 transmembrane segments divided in two groups of 5 and 6, respectively, separated by a large cytoplasmic loop. This predicted topology places the NCKX α-repeats with the same orientation in the plasma membrane. Using thiol-specific drug treatment and site-directed disulfide mapping, we have investigated the orientation of the NCKX2 α-repeats. Our results suggest that the NCKX2 α-repeats have an antiparallel orientation in the plasma membrane. In addition, these experiments suggest that the α-repeats are found in close proximity in the mature configuration of the protein.