SLC9A1 and triple-negative breast carcinoma: We therefore next asked whether spheroid growth of another widely used human breast cancer model, the highly invasive MDA-MB-231 cells which represent the triple-negative breast cancer subtype, was affected by stable knockdown of NHE1, NBCn1, or MCT4 (as noted above, MCT1 is not expressed in MDA-MB-231 at detectable levels, [32]).