Final answer:
A mutation in the retinoblastoma gene causes abnormal growth in retinoblastoma by disrupting the normal regulation of the cell cycle.
Step-by-step explanation:
A mutation in the retinoblastoma gene causes abnormal growth in retinoblastoma. This gene is responsible for producing a protein called the retinoblastoma protein (Rb), which is a tumor suppressor protein that helps control the cell cycle. When the retinoblastoma gene is mutated, it can result in a non-functional or damaged Rb protein, disrupting the normal regulation of the cell cycle and leading to uncontrolled cell division and tumor formation.