Tremella mesenterica (common names include yellow brain, golden jelly fungus, yellow trembler, and witches' butter) is a common jelly fungus in the family Tremellaceae of the Agaricomycotina. The gelatinous, orange-yellow fruit body of the fungus, which can grow up to 7.5 cm (3 in) diameter, has a convoluted or lobed surface that is greasy or slimy when damp.

It is most frequently found on both dead but attached and recently fallen branches, especially of angiosperms, as a parasite of wood-decay fungi in the genus Peniophora. It also grows in crevices in bark, appearing during rainy weather. Within a few days after rain it dries into a thin film or shriveled mass capable of reviving after subsequent rain. This fungus occurs widely in deciduous and mixed forests and is widely distributed in temperate and tropical regions, including those of Eurasia, Africa, Australia, and the Americas. The fungus is potentially edible and produces carbohydrates that have attracted research interest because of their various biological activity.

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