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Oral radiotoxicity is often a limiting factor in cancer treatment. Previously, we demonstrated that transfer of cell-permeable, TAT-fusion Tousled-like kinase 1B (TLK1B) protein in salivary glands effectively mitigates radiation-induced salivary dysfunction. However, similar to most radioprotectors, TLK1B can carry the risk of limiting cancer treatment efficacy. The central goal of the study was, therefore, to reengineer TLK1B as a selective radioprotector of normal cells. Degradation of the extracellular matrix by proteases such as matrix metalloproteinases (MMPs) is a hallmark of aggressive tumors. Increased expression of membrane type 1-MMP (MT1-MMP; also called MMP14) is observed in a variety of cancers including head and neck squamous cell carcinoma (HNSCC). To limit TLK1B transduction to normal cells, we rendered the protein susceptible to MT1-MMP cleavage on the premise that high expression of MT1-MMP on the cell surface of HNSCC will suppress TLK1B internalization. Two optimal MT1-MMP-sensitive sequences (MS) were identified that when incorporated in TAT-TLK1B excluded its cellular entry in HNSCC, SCC40, but not immortalized salivary acinar cells, NS-SV-AC. Importantly, administration of MS-harboring TAT-TLK1B did not affect the sensitivity of tumors to radiation in a nude mouse xenograft tumor model. We conclude that a MMP-sensitive TLK1B can be an attractive therapeutic to allay salivary radiotoxicity without compromising cancer treatment efficacy. Keywords: TLK1, TAT, cell-permeable protein, radiation, radiation protection, DNA repair, salivary glands, MMP, HNSCC