Labrecque MP, Brown LG, Coleman IM, et al. - This study was intended to define two metastatic castration-resistant prostate cancer (mCRPC) phenotypes with Neuroendocrine (NE) features: androgen receptor (AR)-positive NE-positive amphicrine prostate cancer (AMPC) and AR-negative small cell or neuroendocrine prostate cancer (SCNPC). Researchers detected SRRM3 expression in RE1-silencing transcription factor (REST)4-positive, SRRM4-negative AMPC and SCNPC while serine/arginine repetitive matrix protein 4 (SRRM4) was previously implicated in alternative splicing of REST in mCRPC. The outcomes nominate SRRM3 as the principal REST splicing factor expressed in early NE differentiation and provide a framework to molecularly classify diverse NE phenotypes in mCRPC.