However, in a stable genetic configuration in CHO cells, only SEAP-p27 overexpression lead to a significant increase in productivity, with specific SEAP productivity increasing by 15-fold compared to control cells (Fussenegger et al., 1998b) . Intracellular p21 levels were probably insufficiently high to cause significant growth inhibition, and p53-based cell-cycle arrest led to rapid decrease in cell viability accompanied by cell morphologies indicative of apoptosis, even when achieved by overexpression of the apoptosis-deficient mutant p53175P (Rowan et al., 1996) , a phenomenon which could not be observed with p27-induced G1-arrest (Fussenegger et al., 1998b) .