SLC9A1 and neoplasm: However, as strong acute acid-load events such as those induced by NH4Cl pre-pulse are unlikely to be experienced within tumors, it appears that NHE1 activity (to maintain resting pHi) is indeed fundamental for growth in hypoxia as we observed significant decreases with NHE1-ko in both in vitro hypoxia assays and in tumor xenograft experiments.