XYLT1 and Desbuquois dysplasia 2: Molecular defects in XYLT1 have been associated with Desbuquois dysplasia type 2 (DBQD2) with 28 molecularly proven patients reported hitherto [33,34,35,36,37,38,39,40], while XYLT2 mutations cause the so-called spondylo-ocular syndrome (SOS), which has been described in 20 patients thus far [41,42,43,44,45,46].