We consider the so-called robotic cells, i.e. flow-shops with a central robot that carries out the transfers of the parts between the machines. The objective is to describe the robot moves which optimize the throughput rate of a given product. In 1992, a conjecture about the optimal production rate was stated, referred to as the one-cycle conjecture. We shall give the state-of-the-art on this conjecture and add new results on specialized robotic cell arrangements.