Polymerase (RNA) II (DNA directed) Polypeptide J (POLR2J) is a subunit of RNA polymerase II, the polymerase responsible for synthesizing messenger RNA in eukaryotes. The POLR2J protein occurs as a heterodimer with another polymerase subunit; together they develop a core subassembly unit of the polymerase. POLR2J is ubiquitously expressed, having a high expression in the heart and skeletal muscle.