We assessed the effects of ATP6v1c1 on local acidification of the environment by tumor cells, a known factor in breast cancer metastasis [19, 44–46], using acridine orange and found that knockdown blocked the local acidification of the cancer cell microenvironment (Figure 2D-2E); a result in agreement with prior results in osteoclasts knocked down for ATP6v1c1 [18, 43] and indicative of a loss of metastatic potential, where extracellular acidification has been previously shown to be involved in cancer metastatic potential [19, 44–46]. This evidence concerns the gene ATP6V1C1 and breast cancer.