PPi inhibits the formation of apatite crystals [39] as evidenced by hypomorphic mutations of tissue-nonspecific alkaline phosphatase (TNAP) gene (ALPL), which impair PPi hydrolysis and lead to the accumulation of extracellular PPi that cause the soft bones disease known as hypophosphatasia [40,41]. This evidence concerns the gene ALPL and hypophosphatasia.