The finding that anti-PRG4 mAb 4D6, which specifically recognizes PRG4 based on immunoblotting and immunohistochemistry analyses [25,26], interferes with the ability of rhPRG4 to suppress invasive growth of breast cancer cell-derived organoids suggests that mAb 4D6 acts as an PRG4-neutralizing antibody. The gene discussed is PRG4; the disease is breast cancer.