Aspects of the invention provide methods for harnessing the potential of proteins that occur naturally (e.g., in humans) and that have serious but finite toxicity. Aspects of the invention relate to a quantitative systems-biological and structural approach to design a class of chimeric proteins that avoid the toxicity of protein drugs while retaining their desired activities. In particular, chimeric proteins containing a variant form of a natural protein fused to a targeting moiety may be administered to a subject to target a signal (e.g., induction of apoptosis) to particular cells without having a generalized toxic effect in the subject.
