Converts phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P3) to PtdIns-P2. Specific for lipid substrates, inactive towards water soluble inositol phosphates. Defects in INPP5E are the cause of Joubert syndrome type 1 (JBTS1). A disorder presenting with cerebellar ataxia, oculomotor apraxia, hypotonia, neonatal breathing abnormalities and psychomotor delay. Neuroradiologically, it is characterized by cerebellar vermian hypoplasia/aplasia, thickened and reoriented superior cerebellar peduncles, and an abnormally large interpeduncular fossa, giving the appearance of a molar tooth on transaxial slices (molar tooth sign). Additional variable features include retinal dystrophy and renal disease. Defects in INPP5E are the cause of mental retardation- truncal obesity-retinal dystrophy-micropenis (MORMS). An autosomal recessive disorder characterized by moderate mental retardation, truncal obesity, congenital non-progressive retinal dystrophy, and micropenis in males. The phenotype is similar to Bardet-Biedl syndrome and Cohen syndrome Distinguishing features are the age of onset, the non-progressive nature of the visual impairment, lack of dysmorphic facies, skin or gingival infection, microcephaly, mottled retina, polydactyly, and testicular anomalies. Belongs to the inositol 1,4,5-trisphosphate 5- phosphatase type IV family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.