{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"The Retinoblastoma protein (Rb) is a tumor suppressor protein that controls cell cycle progression. Phosphorylation of Rb by cyclin-dependent kinases (CDKs) in response to growth factor signaling releases its inhibition of E2F, allowing cell cycle progression to proceed. Rb-deficient cells may exhibit defects in differentiation and aberrant expression of lineage-specific markers, leading to disrupted tissue development and homeostasis. Mutations in the RB1 gene, or inactivation of Rb through other mechanisms, can lead to uncontrolled cell proliferation and tumor formation."}]}]}