FIG4 (Factor-Induced Gene 4) encodes a dual specificity phosphoinositide 5-phosphatase that plays a critical role in phospholipid metabolism and cellular membrane trafficking 1. The enzyme functions as part of a regulatory complex with PIKfyve and Vac14 to control both synthesis and turnover of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2), catalyzing its dephosphorylation to phosphatidylinositol 3-phosphate while also stimulating PIKfyve's lipid kinase activity 1. FIG4 is essential for normal vesicle trafficking and lysosomal biogenesis, with deficiency leading to abnormal lysosomal storage in neurons 23. Mutations in FIG4 cause multiple autosomal recessive neurological disorders, including Charcot-Marie-Tooth disease type 4J (CMT4J), characterized by motor delay, distal muscle weakness, and both motor and sensory neuropathy 4. Additional FIG4-associated conditions include amyotrophic lateral sclerosis, Yunis-Varon syndrome, and in some cases parkinsonism when specific mutations like I41T are present 5. The clinical manifestations often include cognitive deficits, respiratory difficulties, and nonuniform nerve conduction abnormalities 4. FIG4 deficiency represents a form of lysosomal storage disorder affecting primarily nervous system function, highlighting the critical importance of phosphoinositide signaling in neuronal health 26.