Another genetic defect of renal glucose transport is known to be due to mutations in the GLUT2 transporter, which is responsible for Fanconi-Bickel syndrome (FBS). This is a rare autosomal recessive disorder caused by homozygous or compound heterozygous mutations in the GLUT2 (SLC2A2) gene. It is characterized by hepatorenal glycogen accumulation, proximal renal tubular dysfunction (Fanconi nephropathy), and hypophosphatemic vitamin D dependent rickets with severe growth retardation. The relation between glucose transport and phosphate metabolism is not well understood. Mannstadt et al evaluated two families with varying degrees of hypophosphatemic rickets and urinary phosphate wasting and found novel mutations in GLUT2 as a causative factor for calcium and phosphate imbalances, thought to be due to changes in the phosphate transporter Npt2c. To support this hypothesis, they studied tgGlut2-/- mice who also demonstrated a decrease in Npt2c expression in the proximal renal tubules. While the interplay between this genetic disease and hypophosphatemic rickets is still being elucidated, changes in glucose homeostasis have been shown to have profound effects on bone health and mineral metabolism.