This intriguing conclusion, supported by experimental evidence, suggests that CA9 activity becomes important for the control of \documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\hbox {pH}_i$$\end{document}pHi by tumor cells at critical sizes of the extracellular volume. The gene discussed is CA9; the disease is neoplasm.