In a further preventive measure against possible apoptosis, which was strongly suggested by morphologies of p53 overexpressing cells, we linked SEAP-p27 expression with the expression of the survival gene bcl-x L in a tricistronic configuration (pDD6; Figure 3 ). bcl-x L belongs to the family of bcl-2 anti-apoptosis genes which have been successfully used to suppress apoptosis in production cell lines (Cotter and Al-Rubeai, 1995; Mastrangelo and Betenbaugh, 1998) . Although overexpression of SEAP-p27-bcl-x L induced sustained growth arrest in CHO cells like its dicistronic counterpart, the specific SEAP productivity of arrested cells was increased by an additional factor of three, which corresponds to 30-times higher specific SEAP productivity than respective proliferation-competent control cell lines (Fussenegger et al., 1998a) . This unexpected effect of bcl-x L expression cannot be explained based on the current knowledge of cell-cycle and apoptosis regulatory pathways, and further investigations are needed to reveal the mechanism of this new, apparently apoptosis-unrelated effect of bcl-x L .