3MC syndrome describes a rare developmental disorder, that unifies the overlapping autosomal recessive disorders previously known as Carnevale, Mingarelli, Malpuech and Michels syndromes, characterized by a spectrum of developmental anomalies that include distinctive facial dysmorphism (i.e. hypertelorism, blepharophimosis, blepharoptosis, highly arched eyebrows), cleft lip and/or palate, craniosynostosis, learning disability, radioulnar synostosis and genital and vesicorenal anomalies. Less common features reported include anterior chamber defects, cardiac anomalies (e.g. ventricular septal defect; see this term), caudal appendage, umbilical hernia/omphalocele and diastasis recti.

The presence of a cleft in the oral cavity, the two main types of which are cleft lip and cleft palate. In cleft lip, there is the congenital failure of the maxillary and median nasal processes to fuse, forming a groove or fissure in the lip. In cleft palate, there is a congenital failure of the palate to fuse properly, forming a grooved depression or fissure in the roof of the mouth. Clefts of the lip and palate can occur individually or together. It is preferable to code each defect separately.