Rabbit Anti-Human Rb (phospho Thr821) monoclonal antibody for ICC, IF, WB. Rb, also known as Retinoblastoma, is a tumor suppressor protein that keeps uncontrolled growth and division in check. Rb belongs to the Pocket Protein family whose members have a pocket for binding proteins. It is nuclear protein which is widely expressed in retinal cells. It is inactivated by phosphorylation and activated by dephosphorylation. Phosphorylation of serine 249 and threonine 252 is catalyzed by Cyclin D-cdk4. In humans, the protein is encoded by the RB1 gene located on the q arm of chromosome 13.