Abstract: This disclosure relates to a modified ?-helical bundle cytokine, with reduced activity via an ?-helical bundle cytokine receptor, wherein the ?-helical bundle cytokine is specifically delivered to target cells. Preferably, the ?-helical bundle cytokine is a mutant, more preferably it is a mutant interferon, with low affinity to the interferon receptor, wherein the mutant interferon is specifically delivered to target cells. The targeting is realized by fusion of the modified ?-helical bundle cytokine to a targeting moiety, preferably an antibody. This disclosure relates further to the use of such targeted modified ?-helical bundle cytokine to treat diseases. A preferred embodiment is the use of a targeted mutant interferon, to treat diseases, preferably viral diseases and tumors.