Interphase stage and division stage of the cell are together called cell cycle. The cycle through which cells form, grow and divide is called the cell cycle. According to scientist Lewin, the period between two periodic cell divisions is called cell cycle. According to scientist Karp, the periodic stages between the formation of two cells from one cell through cell division are called cell cycles. Growth factors (growth hormones) bind to receptor proteins on the surface of target cells (cells that will divide) to initiate the cell cycle. In 1953 scientists Howard & Pelc introduced the cell cycle. In 2001, American scientist Leland H. Hartwell discovered the master regulator of the cell cycle.