Classic Ehlers-Danlos syndrome (cEDS) is a heritable connective tissue disorder characterized by skin hyperextensibility, atrophic scarring, and generalized joint hypermobility (GJH). The skin is soft, velvety, or doughy to the touch. In addition, the skin is hyperextensible, meaning that it extends easily and snaps back after release. The skin is fragile, as manifested by splitting of the dermis following relatively minor trauma, especially over pressure points (knees, elbows) and areas prone to trauma (shins, forehead, chin). Wound healing is poor, and stretching, thinning, and pigmentation of scars is characteristic, leading to the presence of atrophic and/or hemosiderotic scars. Easy bruising is also a hallmark of cEDS. GJH is present in most but not all affected individuals, evidenced by the presence of a Beighton score of five or greater, either on examination or historically. Joint instability complications may comprise sprains and dislocations/subluxations. Mild muscle hypotonia with delayed motor development, fatigue and muscle cramps, and some skeletal morphologic alterations (scoliosis, pectus deformities, genus/hallux valgus, pes planus) are regularly observed. While aortic root dilatation and mitral valve prolapse are seen in cEDS, they are rarely clinically significant. Arterial aneurysm and rupture have been reported in a few individuals with cEDS.

Mucopolysaccharidosis type IX (MPS9) is a rare progressive lysosomal storage disorder caused by the deficiency of the enzyme hyaluronoglucosaminidase-1, which degrades hyaluronan (summary by Imundo et al., 2011).

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility and low bone mass. Due to considerable phenotypic variability, Sillence et al. (1979) developed a classification of OI subtypes based on clinical features and disease severity: OI type I, with blue sclerae (166200); perinatal lethal OI type II, also known as congenital OI (166210); OI type III, a progressively deforming form with normal sclerae (259420); and OI type IV, with normal sclerae (166220). Most forms of OI are autosomal dominant with mutations in one of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Glorieux et al. (2000) described a novel autosomal dominant form of OI, which they designated OI type V (OI5), in 7 patients. The disorder was similar to OI type IV but had distinctive clinical, histologic, and molecular characteristics. OI type V is characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation (summary by Cho et al., 2012). OI type V has a variable phenotype. For example, in patients with the more common c.-14C-T variant (614757.0001), distinctive radiographic findings (calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation) are often seen, whereas these findings are not seen in patients with the less common S40L variant (614757.0002).

Cerebellar dysfunction with variable cognitive and behavioral abnormalities (CECBA) is an autosomal dominant neurologic disorder with significant phenotypic heterogeneity, even within families. The disorder is most often diagnosed through genetic analysis with retrospective clinical phenotyping. Symptom onset is usually in early childhood, although later onset, even in adulthood, has been reported. Most affected individuals show global developmental delay from early childhood, particularly of motor and language skills. Many have mild intellectual disability; behavioral and psychiatric abnormalities such as autism and obsessive-compulsive disorder are also often observed. The movement disorder is prominent and may include cerebellar signs such as ataxia, tremor, dysmetria, poor coordination, and dysarthria. Other abnormal movements including spasticity, myoclonus, and dystonia have been reported, thus widening the phenotypic spectrum. Brain imaging is usually normal, but may show cerebellar atrophy or nonspecific white matter lesions. Variable dysmorphic facial features may also be present (summary by Thevenon et al., 2012; Jacobs et al., 2021; Wijnen et al., 2020).