XK aprosencephaly syndrome is a very rare syndromic type of cerebral malformation characterized by aprosencephaly (absence of telencephalon and diencephalon), oculo-facial anomalies (i.e. ocular hypotelorism or cyclopia, malformation/absence of nasal structures, cleft lip), preaxial limb defects (i.e. hypoplastic hands, absent halluces) and various other anomalies including ambiguous genitalia, imperforate anus, and vertebral anomalies. The syndrome is thought to have an autosomal recessive mode of inheritance.

Missing or malformed long bones of the extremities with the distal parts (such as hands and/or feet) connected to the variably shortened or even absent extremity, leading to a flipper-like appearance, as opposed to other forms of limb malformations were either the hole limb is missing (such as amelia), or the distal part of a limb is absent (peromelia).