Belderbos BPS, Sieuwerts AM, Oomen-de Hoop E, et al. - Researchers sought to assess the clinical value of AR-V7 via determining the association of AR-V7 detection in circulating tumour cells (CTCs) in patients with metastatic castration-resistant prostate cancer (mCRPC) with CTC counts and survival. They used data of three prospective clinical trials in which clinical data of patients with mCRPC, who progressed after docetaxel and/or enzalutamide or abiraterone were collected and CTC count and AR-V7 expression were determined in blood samples withdrawn at baseline and follow-up. They identified a total of 127 samples as evaluable for the analysis of CTC count vs AR-V7 status. The analysis revealed no additional prognostic value of AR-V7 expression in CTCs in patients with mCRPC, mostly treated with cabazitaxel. A predictive biomarker remain an important unmet medical need in patients with mCRPC with a predefined worse prognosis of a higher CTC count (≥ 5).