Beta-barrel protein that binds unconjugated bilirubin with high affinity. Excitation of the bilirubin-bound protein gives rise to green fluorescence, both under normoxia and hypoxia. The apoprotein is not fluorescent. Does not emit fluorescence in the presence of ditauro-bilirubin, urobilin or biliverdin. Excitation of the bilirubin-bound protein gives rise to green fluorescence, with maximal fluorecence emission at 527 nm. Monomer. Detected in small-diameter muscle fibers from the white muscle layer from juvenile animals (glass eels) (at protein level). Detected in small-diameter muscle fibers from juvenile animals (glass eels). Forms a beta-barrel structure that accommodates bilirubin in its interior. Becomes fluorescent upon binding free, unconjugated bilirubin, a heme metabolite. Can be used as diagnostic tool to detect abnormally high levels of free bilirubin, which are indicative of impaired liver function. Could also be used in research as fluorescent marker in eukaryotic cells and animals. Belongs to the calycin superfamily. Fatty-acid binding protein (FABP) family.