The tumor microenvironment is acidic. Image shows ZR75.1 breast cancer tumor growing in a dorsal window chamber. Twenty-four hours prior to imaging, mice were injected with Alexa546 dye conjugated to a pH-low inserting peptide, pHLIP (courtesy of O. Andreev and Y. Reshentnyak, University of Rhode Island, Kingston, RI), showing uptake in the tumor and surrounding stroma. Neutralization of this acidity with oral buffers can improve response to checkpoint inhibitors and adoptive T-cell transfer immune therapies, as described in this issue. (Image courtesy of V. Estrella). For details, see article by Pilon-Thomas and colleagues on page 1381.