SLC9A1 and cancer: As the most important transport systems in RVI are NKCC1, NHE1, the Na+/K+ ATPase, and in some cells also ENaC type cation channels (Hoffmann et al., 2009), it seems likely that increased expression or function of these in epithelial cancer would render tumor cells resistant to apoptosis, and in fact, this has been demonstrated in several types of cancers (see below).