Although rare in primary prostate cancers,5-7AR gene alterations are highly prevalent in mCRPC.8-13 Metastatic tissue biopsies as a sole means to detect and observe changes in AR status is impractical, and thus cell-free DNA (cfDNA) is gaining traction as a minimally invasive and easily obtainable tumor biopsy surrogate. The gene discussed is AR; the disease is neoplasm.