The Twirler mouse, a model for the study of cleft lip and palate.

Twirler (Tw) is a semidominant mutation in the mouse affecting the embryonic development of the midfacial region. Most heterozygous Tw mice, +/-, become obese at adulthood with a concomitant decrease in fertility. Homozygous mice have clefts of the midfacial region and a disrupted nasal cavity. Midfacial clefts included clefts of the palate combined with either unilateral or bilateral clefts of the lip. The clefts of the lip were either complete or incomplete. The palatal shelves in Tw/Tw were very much reduced. Apart from these defects, homozygous Tw looked normal, and were born alive, although they reportedly die within 24 h after birth. It is proposed that the Twirler model can be used to improve understanding of the genetic mechanisms involved in the normal development of the midfacial region.