Hyccin is a 521 amino acid cytoplasmic protein that is widely expressed with highest levels found in heart, brain, placenta, spleen and testis. Belonging to the FAM126 family, hyccin may play a role in the ∫-catenin/Lef signaling pathway. Hyccin is likely involved in the process of myelination of the central and peripheral nervous system. Defects in the gene encoding hyccin are the cause of leukodystrophy hypomyelinating type 5 (HLD5), which is characterized by congenital cataract, progressive neurologic impairment and diffuse myelin deficiency. Individuals affected by HLD5 experience progressive pyramidal and cerebellar dysfunction along with muscle weakness in the lower limbs. Hyccin exists as two alternatively spliced isoforms and is encoded by a gene located on human chromosome 7.