SLC9A1 and neoplasm: To address the question of whether Rho proteins are involved in the regulation of tumour cell NHE1 activity, we compared the effect of transient transfection of HA-tagged dn (Fig. 4a) or constitutively active (ca; Fig. 4b) mutants of Rho family proteins on both the basal activity of the NHE1 and its activation by serum deprivation.