Recently, there is a growing interest in the intracellular signal-targeting gene therapy or diagnosis, mainly by using the reaction of targeting enzymes with peptide substrates. In the present study, we proved the importance of target intracellular signal-specificity peptide substrate for intracellular signals-targeting gene therapy or diagnosis. Protein kinase C (PKC) was used as a trigger to activate the transgene expression. Two peptides, a positive peptide showing phosphorylation levels on several PKC isozymes (PKCalpha, betaII, gamma, epsilon, eta, zeta, and iota/lambda) and a negative peptide in which the phosphorylation site was destroyed by changing from serine to alanine, were designed. Moreover, two polymers possessing each peptide as a pendant chain, a PKC-responsive conjugate [PPC(S)] and a negative control conjugate [PPC(A)], were synthesized. After the introduction of complexes into cells or tissues, gene expression for PPC(S)/DNA complexes was higher that for PPC(A)/DNA complexes. However, no difference in gene expression between B16 melanoma tumors and normal skin tissues was identified. These results suggest that a peptide substrate specific to a target intracellular signal is very important for intracellular signals-targeting gene therapy or diagnosis.