Genes that make mosquitoes glow under ultraviolet light help scientists measure the spread of transgenic mosquitoes after they’ve been released to suppress wild populations of vector mosquito species. A recent study published in the Journal of Medical Entomology identifies a new promoter gene for turning on fluorescent protein production in Aedes aegypti mosquitoes. Here, two mosquito larvae are shown; the larva on the right possesses the fluorescent gene marker and glows clearly, while the larva on the left, lacking the fluorescent gene marker (and noted by the arrows), is barely visible. (Image originally published in Webster and Scott 2021, Journal of Medical Entomology)