The cochlea in the mammalian inner ear is a sensitive and sharply organized sound-detecting structure. The proper specification of neurosensory-competent domain in the otic epithelium is required for the formation of mature neuronal and sensory domains. Genetic studies have provided many insights into inner ear development, but there have been few epigenetic studies of inner ear development. CTCF is an epigenetic factor that plays a pivotal role in the organization of global chromatin conformation. To determine the role of CTCF in the otic sensory formation, we made a conditional knockout of Ctcf in the developing otic epithelium by crossing Ctcf(fl/fl) mice with Pax2-Cre mice. Ctcf deficiency resulted in extra rows of auditory hair cells in the shortened cochlea on mouse embryonic day 14.5 (E14.5) and E17.5. The massive and ectopic expression of sensory specifiers such as Jagl and Sox2 indicated that the sensory domain was expanded in the Ctcf-deficient cochlea. Other regulators of the sensory domain such as Bmp4, Gata3, and Fgf10 were not affected. These results suggest that CTCF plays a role in the regulation of the sensory domain in mammalian cochlear development. (C) 2019 Elsevier Inc. All rights reserved.