Opalin also called Tmem10 is a transmembrane protein detected specifically in mammalian oligodendrocytes. Opalin homologs are found only in mammals and not in the genome sequences of other animal classes (1). Deficient mice are born the expected Mendelian ratio and have a normal body shape and weight. No obvious abnormalities in gross anatomy or major myelin protein components nor in the compartment of myelinated axons and oligodendrocyte maturation. Nevertheless, deficient mice display increased exploratory activity in a novel environment (2).