ATR stands for all-trans-retinal which is just another name for vitamin A. ATR is required for proper phototransduction across species. It binds to opsins to form rhodopsins and only then does it become sensitive to light. Drosophila do not endogenously produce vitamin A, so we must supplement it in their diet (ATR preparation)
molecular weight: 284.44 g/mol
supplier: sigma
references
Wang, T., & Montell, C. (2005). Rhodopsin Formation in Drosophila Is Dependent on the PINTA Retinoid-Binding Protein. The Journal of Neuroscience, 25(21), 5187–5194. https://doi.org/10.1523/JNEUROSCI.0995-05.2005