In vitro phototoxicity tests measure skin irritation responses to agents without the need for an immune response. This is accomplished by comparing the capacity for cell cultures to display a dye that is visible under UVA light in the presence or absence of the cytotoxic compound in question. Such tests replace the need to test substances in vivo. The European Union validated the 3T3 Neutral Red Uptake (NRU) Phototoxicity Assay in 2000, marking a significant shift away from in vivo testing towards in vitro alternatives.
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