RPB1 (RNA polymerase II subunit B1) is the catalytic component of RNA polymerase II which synthesizes mRNA and non-coding RNAs. During transcription, elongation is influenced by the phosphorylation status of the C-terminal domain (CTD) of RPB1, which acts as an assembly platform for factors regulating transcription initiation, elongation, termination and mRNA processing. Phosphorylation occurs mainly at residues 'Ser-2' and 'Ser-5' of the tandem 7 residue repeats in the C-terminal domain (CTD), which activates Pol II. This phosphorylation also can occur at 'Ser-7' of the heptapepdtide repeat. This antibody recognizes the 'Ser-7' CTD residue of RPB1.