To investigate the potential photocytotoxicity of dTAT1 to cells, we conducted in vitro cell viability experiments using the PrestoBlue assay on a Human Epidermoid Carcinoma (A-431) cell line. Photocytotoxicity was evaluated by measuring cell viability following photodynamic treatment with a lowdose UVA irradiation (5 J cm -2 ) delivered via a 370 ± 9 nm LED lamp. Controls included non-irradiated cells treated with dTAT1 to assess dark cytotoxicity and untreated cells irradiated with UVA to evaluate any UVA-induced damage. The cell viability results, presented in Figure , indicate that dTAT1 is noncytotoxic at concentrations up to 200 µM, under both light and dark conditions. Importantly, the absence of photocytotoxicity and cytotoxicity underscores dTAT1's safety profile, making it suitable for cellular incorporation with minimal cell death. These findings further support dTAT1's potential as a superior candidate for expanding the genetic alphabet, 8 enabling its use without compromising cell viability.