C4B and Ehlers-Danlos syndrome: Deficiency and haploinsuffiency of tenascin-X, a large extracellular glycoprotein, has been identified as a cause of joint hypermobility in patients with benign joint hypermobility syndrome and the mobility type-Ehlers-Danlos syndrome.14 Tenascin-X forms a genetic unit with CYP21B, C4A, and C4B, termed the RCCX module.