The SLC9A1 gene, the Na+/H+ exchanger isoform 1 is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium. The human protein is 815 amino acids in length. Five hundred N-terminal amino acids make up the transport domain of the protein and are believed to form 12 transmembrane segments. Recently, a genetic mutation of the Na+/H+ exchanger isoform 1, N266H, was discovered in a human patient through exome sequencing. We examined the effect of this mutation on expression, targeting and activity of the Na+/H+ exchanger. Mutant N266H protein was expressed in AP-1 cells, which lack their endogenous Na+/H+ exchanger protein. Targeting of the mutant protein to the cell surface was normal and expression levels were only slightly reduced relative to the wild type protein. However, the N266H mutant protein had no detectable Na+/H+ exchanger activity. A histidine residue at this location may disrupt the cation binding site or the pore of the Na+/H+ exchanger protein.