Scientists hope that a handful of monkeys that glow—in the ultraviolet (UV)—will help shed light on human genetic diseases. A study, published today in Nature, reports that for the first time an inserted gene—in this case, one that makes parts of the marmosets glow green under the UV rays—was passed along to a subsequent generation. (Earlier transgenic monkeys failed to produce offspring that carried the modified luminescent DNA.) Study authors note that it is an important step forward in studying hereditary genetic diseases, such as Parkinson's. Shown here, twins Kei and Kou, with UV images of their feet, were two of the five transgenic marmosets born in the study.