Serine/Arginine Repetitive Matrix 4 (SRRM4), also known as nSR100, is a vertebrate- and neural-specific Ser/Arg repeat-related protein that acts in the brain as a key regulator of alternative splicing. nSR100 has been identified as an activator of the small cell lung cancer-specific isoform of RE1-silencing transcription factor. The protein is involved in the progression of neuroendocrine prostate cancer, an aggressive subtype of castration-resistant prostate cancer, and has been suggested as a potential therapeutic target. A correlation has been made between underexpression of the SRRM4 protein and autism spectrum disorders.