Previous data have shown that inhibiting V-ATPases inhibits invasion [9, 10] and our experiments indicate that inhibiting V-ATPases decreases the activity of secreted cathepsin B. Activating cathepsin B in the extracellular space may be part of the mechanism by which cell-surface V-ATPases are enhancing invasion of breast cancer cells. The gene discussed is CTSB; the disease is breast cancer.