Sprouty was first identified in Drosophila as a novel antagonist of the fibroblast growth factor signalling pathway. Sprouty proteins comprise a big family, members of which are characterized by a cysteine-rich domain which confers inhibitory activity, whereas differences in the N-terminal region may be responsible for functional divergence. The role of Sprouty in RTK (receptor tyrosine kinase) signalling pathways is still controversial. Sprouty may negatively or positively regulate RTK signalling via differential interaction with different signalling molecules, and hence exert different mechanism of action.