PROS1 and cystic fibrosis: We previously showed that the activation of cell cycle checkpoints under HU-induced replication stress, overriding checkpoints during CF-induced PCC (under prolonged replication stress), was a multistage process requiring the participation of many proteins, and included: (i) detection of an abnormality (by sensory protein factors); (ii) transfer of a signal (through a signal transmission factors) and (iii) initiation of adequate responses (e.g. cell cycle arrest, DNA repair, etc.)via effector factors.