PROS1 and breast cancer: The human pro-apoptotic protein tBid is the effector molecule of our suicide vector pLtBid(INS)2R. tBid fulfills several qualifications of a suitable suicide protein: i) cell death is induced within hours after expression of tBid, ii) tBid is efficient in very low concentrations [23], iii) it is not immunogenic as, for instance, viral suicide proteins [11,32], and iv) it has been successfully applied as suicide protein in a gene therapeutic approach where breast cancer cells were specifically killed by tBid in vitro [24].