NaPi-Ilb encodes a Na+-dependent Pi co-transporter, which is expressed in various adult tissues and mediates transport of extracellular Pi ions coupling with Na+ ion. To define the role of NaPi-Ilb in vivo, NaPi-IIb gene deficient mice were generated utilizing targeted mutagenesis, yielding viable, heterozygous NaPi-IIb mice. In contrast, homozygous NaPi-IIb, mice died in utero soon after implantation, indicating that NaPi-IIb was essential for early embryonic development. In situ hybridization revealed NaM-IIb, mRNA expression in the parietal endoderm, followed by the visceral endoderm, at a time point prior to establishment of a functioning chorio-allantoic placenta. At the time point of functional placenta development, the main site of NaPi-Ilb production resided in the labyrinthine zone, where embryonic and maternal circulations were in closest contact. Expression patterns of NaPi-IIb suggest that NaPi-Ilb plays an important role in Pi absorption from maternal circulation. (C) 2009 Published by Elsevier Inc