Abstract: The present invention relates to fusion proteins in which a pharmaceutically active component is fused to an antibody mimetic. The invention specifically concerns fusion proteins comprising interferons or biologically active muteins thereof and modified hetero-dimeric ubiquitin proteins as specific targeting domain. The invention further relates to these fusion proteins for use in medicine, in particular for use in the treatment of cancer or infectious diseases. The invention is further directed to pharmaceutical compositions comprising a pharmaceutically acceptable carrier in combination with such fusion proteins, and in combination with cancer therapeutic agents. Moreover, the invention relates to a method for the generation of said fusion proteins.