The alpha-complex proteins (CP) are generally known as RNA-binding proteins that interact in a sequence-specific fashion with single-stranded poly(C). These proteins are mainly involved in various post-transcriptional regulations (e.g., mRNA stabilization or translational activation/silencing). Here we report a novel function of CP3, a member of the CP family. CP3 bound to the double-stranded poly(C) element essential for the mu opioid receptor (MOR) promoter and repressed the promoter activity at the transcriptional level. We identified CP3 using affinity column chromatography containing the double-stranded poly(C) element and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. CP3 binding to the poly(C) sequence of the MOR gene was sequence specific, as confirmed by the supershift assay. In cotransfection studies, CP3 repressed the MOR promoter only when the poly(C) sequence was intact. Ectopic expression of CP3 led to repression of the endogenous MOR transcripts in NS20Y cells. When CP3 was disrupted using small interfering RNA (siRNA) in NS20Y cells, the transcription of the endogenous target MOR gene was increased significantly. Our data suggest that CP3 can function as a repressor of MOR transcription dependent on the MOR poly(C) sequence. We demonstrate for the first time a role of CP3 as a transcriptional repressor in MOR gene regulation.--Choi, H. S., Kim, C. S., Hwang, C. K., Song, K. Y., Law, P.-Y., Wei, L.-N., and Loh, H. H. Novel function of the poly(C)-binding protein CP3 as a transcriptional repressor of the mu opioid receptor gene.