LNS is due to mutations in the HPRT1 gene,[REF][REF] so named because it codes for the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT or HGPRT, ).
If a woman is the first in her family with an affected son, Haldane's rule predicts a 2/3 chance that she is a carrier and a 1/3 chance that the son has a new germline mutation.