Synaptojanin 1 (SYNJ1) is a phosphoinositide phosphatase highly expressed in nerve terminals. Its two phosphatase domains dephosphorylate phosphoinositides present in membranes , while its proline-rich domain directs protein-protein interactions with synaptic components , leading to efficient recycling of synaptic vesicles in neurons. Triplication of SYNJ1 in Down ' s syndrome is responsible for higher level of phosphoinositides , enlarged endosomes , and learning deficits. SYNJ1 downregulation in Alzheimer ' s disease models is protective towards amyloid-beta peptide (Aí µí»½) toxicity. One missense mutation in one of SYNJ1 functional domains was recently incriminated in an autosomal recessive form of early-onset Parkinson ' s disease (PD). In the third decade of life , these patients develop progressive Parkinsonism with bradykinesia , dystonia , and variable atypical symptoms such as cognitive decline , seizures , and eyelid apraxia. The identification of this new gene , together with the fact that most of the known PD proteins play a role in synaptic vesicle recycling and lipid metabolism , points out that synaptic maintenance is a key player in PD pathological mechanisms. Studying PD genes as a network regulating synaptic activity could bring insight into understanding the neuropathological processes of PD and help identify new genes at fault in this devastating disorder .