ENPP1 and hypophosphatemic rickets, autosomal recessive, 2: Also, increasing serum FGF23 concentration could also result from the Autosomal Recessive Hypophosphatemic Rickets (ARHR), including 1) Autosomal Recessive Hypophosphatemic Rickets Type 1 (ARHR1), which is caused by inactive mutation in the dentin matrix acidic phosphoprotein (DMP1) gene acting on the inhibition of FGF23 expression, and 2) Autosomal Recessive Hypophosphatemic Rickets Type 2 (ARHR2), which is caused by ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) inactive mutation (Lorenz-Depiereux et al., 2006).