Fluorescent proteins (FPs) are powerful tools for visualization of molecular and cellular processes; however, most FPs lose fluorescence at a pH lower than their neutral pKa (~6). A team of researchers developed the acid-tolerant green FP -- termed Gamillus -- cloned from flower hat jellyfish. Gamillus exhibits excellent brightness, maturation speed, and photostablity, even in low pH environments, making it a feasible molecular tag for imaging in acidic organelles.