The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

Classic Schinzel-Giedion syndrome (SGS), an ultra-rare multisystem disorder caused by gain-of-function pathogenic variants in a SETBP1 mutational hot spot, is characterized by global neurodevelopmental impairment leading to moderate-to-profound intellectual disability, epilepsy (often refractory to treatment), hypotonia, spasticity, dysautonomia, hearing loss, and cerebral visual impairment. Other findings can include poor weight gain often associated with gastroesophageal reflux disease, chronic vomiting, constipation, gastroparesis, and/or feeding intolerance. Structural malformations can involve the heart, skeleton, kidney and urinary tract, genitalia, and brain. Anomalies of the liver, spleen, and/or pancreas are less common. Other features may include neuroepithelial neoplasia, severely disrupted sleep, choanal stenosis, inguinal hernia, sensitive skin, and increased risk of infection. To date, more than 50 individuals have been reported with molecularly confirmed classic SGS. Atypical SGS, reported in five individuals to date, is caused by pathogenic SETBP1 variants in proximity to – but not within – the mutational hot spot. The broad spectrum of clinical features of variable severity partially overlaps with classic SGS; however, this spectrum does not include risk for neuroepithelial neoplasia to date.

Teebi-Shaltout syndrome is characterized by slow hair growth, scaphocephaly with prominent forehead, bitemporal depression, absence of primary teeth, camptodactyly, and caudal appendage with sacral dimple (summary by Aldemir et al., 2013).

Heterotaxy ('heter' meaning 'other' and 'taxy' meaning 'arrangement'), or situs ambiguus, is a developmental condition characterized by randomization of the placement of visceral organs, including the heart, lungs, liver, spleen, and stomach. The organs are oriented randomly with respect to the left-right axis and with respect to one another (Srivastava, 1997). Heterotaxy is a clinically and genetically heterogeneous disorder. For a discussion of genetic heterogeneity of visceral heterotaxy, see HTX1 (306955).

Multiple congenital anomalies-hypotonia-seizures syndrome is an autosomal recessive disorder characterized by neonatal hypotonia, lack of psychomotor development, seizures, dysmorphic features, and variable congenital anomalies involving the cardiac, urinary, and gastrointestinal systems. Most affected individuals die before 3 years of age (summary by Maydan et al., 2011). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).