TMEM231 is a transmembrane protein essential for ciliary transition zone organization and function. As a core component of the MKS (Meckel-Gruber syndrome) complex, TMEM231 localizes to the transition zone between the basal body and axoneme, where it works cooperatively with B9D1 and CC2D2A to form a diffusion barrier 1. This barrier maintains the cilium as a compartmentalized signaling organelle by restricting plasma membrane protein diffusion into the ciliary membrane 12. TMEM231 is critical for ciliogenesis and sonic hedgehog signaling; disruption impairs localization of signaling receptors like Arl13b and Inpp5e to cilia 2. Pathogenic TMEM231 variants cause ciliopathies with overlapping phenotypes, including Meckel syndrome (MKS) and Joubert syndrome (JBTS) 34. MKS manifests as a severe perinatally-fatal disorder with polycystic kidneys, occipital encephalocele, polydactyly, liver fibrosis, and CNS malformations 3. JBTS presents with cerebellar vermis hypoplasia, oculomotor apraxia, and variable renal/ocular involvement 4. Mutations range from splice-site variants causing exon skipping to missense changes, with genotype-phenotype correlation observed within families 35. Clinical diagnosis requires genetic testing, with implications for genetic counseling and prenatal diagnosis in affected families.