Doctors might one day remove cancerous tumors with the help of a spray-on fluorescent indicator, according to a report (Sci. Transl. Med., DOI: 10.1126/scitranslmed.3002823). Within minutes of being sprayed, cancer cells in suspect tissue would emit fluorescent light, guiding surgeons to the locations of malignancies. Although other fluorescent probes are in development for surgery, the new indicator acts more quickly and can be applied on the spot. Hisataka Kobayashi of the U.S.’s National Cancer Institute, Yasuteru Urano of Japan’s University of Tokyo, and coworkers demonstrated that their indicator, a caged γ-glutamyl hydroxymethyl rhodamine green compound, lit up tumors in mice with ovarian cancer within 90 seconds of being applied. The probe is nonfluorescent until it comes into contact with cancer cells, many of which overexpress the enzyme γ-glutamyltranspeptidase on their surfaces. The enzyme uncages the indicator by cleaving the glutamate group, causing it to fluoresce.