Methods for determination of steroids and abiraterone in serum and prostate tissue samples by mass spectrometry were as previously reported.(21 , 22 ) Briefly, frozen needle biopsy tissue cores were weighed, added to 60C water containing deuterated internal standards, heated to 60C for 10 minutes, and homogenized using a tissue homogenizer (Precellys; Bertin, Rockville, MD, USA); supernatant was extracted twice with hexane (ethyl acetate [80:20 v/v]), and the organic layer was dried (SpeedVac; Thermo Scientific, Waltham, MA, USA), derivatized with 0.025 M hydroxylamine hydrochloride for 24 hours at room temperature to form oximes, and quantified using liquid chromatography electrospray-ionization tandem mass spectrometry. Lower limits of detection and quantitation (LLOD and LLOQ) for steroids in serum were 0.49 pg/sample (0.02 ng/ml) for progesterone, androstenedione (AED), and testosterone; 0.98 pg/sample (0.04 ng/ml) for pregnenolone and dihydrotestosterone (DHT)\; 3.9 pg/sample (1.2 ng/ml) for DHEA. The lower limit of detection for steroids in tissue was 0.49 pg/sample (0.02 pg/mg) for progesterone, AED, DHT, and testosterone; 0.98 pg/sample (0.04 pg/mg) for DHEA; and1.96 pg/sample (0.08 pg/mg) for pregnenolone. The LLOD and LLOQ for abiraterone in serum was 0.06 ng/sample (0.005 ng/dL) and in tissue was 1.2 ng/sample (0.06 pg/gm). Additional information on assay methodology is provided in Supplementary Figure 1.