As previously noted by Swietach et al.11\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 regulation is not affected by CA9 expression in isolated tumor cells, but its role becomes important when cells are grown as three-dimensional aggregates (tumor spheroids). The gene discussed is CA9; the disease is neoplasm.