The substantial sensitivity of pHi to extracellular acidification also in breast cancer cells (Figure 4G,H) is consistent with previous findings from human and murine breast cancer tissue [8,9,10] and is likely explained by the prominent inhibition of NBCn1 (~60%) and NHE1 (~85%) activity when pHo is lowered to 6.8 [61]. This evidence concerns the gene SLC9A1 and breast carcinoma.