To investigate whether cleaved CPP-Dox is able to penetrate and kill even Dox-resistant cancer cells, comparisons of cytotoxicities and uptake rates of MMP-cleaved CPP-Dox were made between Dox-resistant cells (NCI/ADR, MES-SA/Dx5) and Dox-sensitive cells (MCF7, MES-SA). Here, CPP is linked to cancer.