Osteocalcin is the most abundant noncollagenous protein of bone. Here we report that the mouse genome contains an osteocalcin cluster composed of three genes arranged within a 23-kilobase span of genomic DNA. We named them osteocalcin gene 1 (OG1), osteocalcin gene 2 (OG2), and osteocalcin-related gene (ORG) in order from the 5' end to the 3' end of the cluster. Hybridization of polymerase chain reaction-amplified cDNAs with specific oligonucleotides and RNase protection assays showed that OG1 and OG2 are expressed only in bone, whereas ORG is transcribed in kidney but not in bone. Furthermore, during embryogenesis, OG1 and OG2 begin to be expressed at day 15.5, while ORG is transcribed as early as day 10.5. The protein encoded by ORG has a similar pattern of expression and identical structural features to nephrocalcin, a calcium-binding protein partially purified from kidney that plays a role in calcium reabsorption and in prevention of nephrolithiasis. The nephrocalcin gene has not been cloned in any species; we propose that ORG is the mouse nephrocalcin gene. The existence of several osteocalcin or osteocalcin-related sequences is not restricted to mouse but is present in every species we examined.