CD9 and prostate cancer: During the progression of prostate cancer, CD9 expression is significantly reduced or even lost due to the loss or mutation of its transcript, indicating that CD9 inactivation plays an important role in the disease’s progression (21) Notably, the expression of CD9 protein is lower in prostate cancer compared to benign prostatic hyperplasia, and this downregulation is associated with a shorter progression-free survival (PFS) in patients with high-grade prostate cancer.