Background/Aims: The osteocalcin-related gene (ORG) is a mouse-specific member of the osteocalcin gene cluster predicted to encode a γ-carboxyglutamic acid-rich protein. ORG mRNA has been predicted to encode nephrocalcin and shown to be expressed in the kidney where it could serve as an important crystallization inhibitor. To determine whether ORG encodes mouse nephrocalcin, we investigated its in vivo and in vitro expression. Methods: We expressed fluorescent fusion ORG proteins in kidney cell lines and generated transgenic mice expressing enhanced green fluorescent protein under the control of the ORG promoter. Results: ORG mRNA was shown to be expressed in mouse kidneys and in a variety of other tissues. Fusion constructs transfected in opossum kidney cells demonstrated integrity of the open reading frame with the presence of a protein of the expected molecular weight. However, kidneys from transgenic mice carrying the enhanced green fluorescent protein gene under the control of the ORG promoter (5.8 kb fragment) demonstrated no expression of the transgene in kidneys or other tissues. Conclusion: We conclude that ORG, the third gene of the mouse osteocalcin gene cluster is silent and unlikely to play a major role in mouse renal physiology.