Core component of RNA polymerase II (Pol II), a DNA-dependent RNA polymerase which synthesizes mRNA precursors and many functional non-coding RNAs using the four ribonucleoside triphosphates as substrates. {ECO:0000269|PubMed:27193682, ECO:0000269|PubMed:30190596, ECO:0000269|PubMed:37595871, ECO:0000269|PubMed:9852112}. This is the function of POLR2J (RNA polymerase II subunit J, ENSG00000005075).