Liquid biopsy of exosomes isolated from patients with prostate cancer revealed that exosomes are enriched for genes that are hallmarks of prostate cancer, such as androgen receptor, kallikreins (KLK2), cyclin-dependent kinase inhibitor 1A (CDKN1A), KLK10, JUN, and B2M (β2 microglobulin). This evidence concerns the gene KLK4 and prostate cancer.