Rb gene was the first-ever tumor suppressor gene discovered. Loss of function mutations in the Rb gene leads to retinoblastoma – cancer of the retinal cells of the eye. A healthy individual possesses two functional copies of the Rb gene, which encode a retinoblastoma protein that regulates cell cycle progression. In the case of hereditary retinoblastoma, a child inherits one functional and one mutated Rb gene from their parents. Every somatic cell of these individuals will therefore have only one normal Rb gene, but the amount of Rb protein produced is enough to maintain the normal cell cycle. However, if any of these somatic cells lose the other functional copy of the gene, the cell stops producing the Rb protein altogether and becomes cancerous. In rare cases, even when an individual inherits two functional Rb gene copies, both copies of the Rb gene can be lost or inactivated in a single cell lineage by two independent events over time. Such double loss of function mutations in the retinal cells leads to non-hereditary or sporadic retinoblastoma.