Complex sentence: One function of RB is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide, until it is phosphorylated to pRb where it becomes inactive and allows cell cycle progression.
Split into simpler sentences: One function of Rb is to prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide. When the cell is ready to divide, Rb is phosphorylated to pRb where it becomes inactive and allows cell cycle progression.