In the majority of hypophosphatasia patients, reductions in the serum lev-els of alkaline phosphatase activity are caused by various missense muta-tions in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. A unique frame-shift mutation due to a deletion of T at cDNA number 1559 [TNSALP (1559delT)] has been reported only in Japanese patients with high allele frequency.