The present invention relates to masked IL-12 cytokines, comprising an IL-12 cytokine or functional fragment thereof, a masking moiety and a proteolytically cleavable linker. The masking moiety masks the IL-12 cytokine or functional fragment thereof thereby reducing or preventing binding of the IL-cytokine or functional fragment thereof to its cognate receptor, but upon proteolytic cleavage of the cleavable linker at a target site, the IL-12 cytokine or functional fragment thereof becomes activated, which renders it capable or more capable of binding to its cognate receptor.