TTD is part of a more broadly defined group of diseases identified as IBIDS (ichthyosis, brittle hair, impaired intelligence, decreased fertility and short stature). Photosensitive cases are also identified as PIBIDS (photosensitivity with IBIDS). Cases without manifest ichthyosis are also identified as PBIDS. These syndromes defy rigorous definition because of clinical variation between patients. The original two cases were described by Tay in oriental siblings, whose parents were first cousins; thus the disease is also known as Tay syndrome.