The recent manuscript in New England Journal of Medicine by Antonarakis et al. [1] has important clinical implications. This study evaluates mRNA expression of a particular androgen receptor splice variant-7 (AR-V7), in circulating tumor cells (CTCs) from metastatic castrate-resistant prostate cancer (mCRPC) patients receiving enzalutamide or abiraterone. The findings were striking, none of the 18 patients with detectable AR-V7 in CTCs had prostate-specific antigen (PSA) responses. Further, the median time to PSA progression after enzalutamide or abiraterone treatment was only 1.3-1.4 months in AR-V7-positive patients as compared to 5.3-6.1 months in AR-V7 negative patients. AR-V7 in CTCs was also associated with shorter survival.