Ectopic calcification can cause pain and limit mobility. Studies suggest that circadian genes may play a role in the calcification process. Core circadian genes Clock, Npas2, and Bmal1 are transcription factors that form CLOCK:BMAL1 or NPAS2:BMAL1 transactivator complexes that drive the rhythmic expression of circadian oscillator genes and output genes. Circadian oscillator genes Period1-3 and Cryptochrome1-2 encode proteins that form transcription repressor complexes that feedback to inhibit CLOCK/NPAS2:BMAL1 activity, thus completing the feedback loop that is the basis of the molecular circadian clockwork. Arrhythmic Bmal1-/- mice exhibit site-specific, age-dependent arthropathy. While studying the circadian phenotype of Clock-/-;Npas2m/m double mutant mice, we discovered that these double mutant mice develop site-specific arthropathy similar to the arthropathy described in Bmal1-/- mice. Based on the circadian clockwork mechanism, we hypothesized that CLOCK/NPAS2:BMAL1 transactivator complexes drive the expression of a gene (or genes) that prevents age-dependent arthropathy. To investigate Clock-/-;Npas2m/m double mutant mouse arthropathy, we evaluated mutant mice using X-ray, micro-computed tomography, and histology, and found that Clock-/-;Npas2m/m double mutant mice exhibit age-dependent, site-specific arthropathy that phenocopies that of Bmal1-/- mice. The costosternal junction and calcaneal tendon are most prominently affected, in that calcification of those tissues is detectable as early as 4-5 weeks and 11-12 weeks, respectively. The arthropathic lesions in these tissues consist of calcium phosphate vii deposits, and in Bmal1-/- costosternal junction calcifications, the deposits contain calcium pyrophosphate dihydrate crystals. Mechanical stress, disregulation of centrally-regulated circadian rhythms, and systemic serum mineral imbalances likely do not contribute to this pathology. In vitro micromass cultures generated from Clock-/-;Npas2m/m double mutant mouse embryonic fibroblasts do not exhibit irregular chondrocyte differentiation compared to wild-type cultures, suggesting that chondrocyte cell-autonomous mechanisms are insufficient to induce this arthropathy. Analysis of Clock-/-;Npas2m/m double mutant intersternebral tissue RNA did not reveal significant changes in chondrocyte or calcification-related gene expression. Histological stains showed an absence of osteoblasts and osteoclasts around costosternal junction calcifications, suggesting that these cell types are not contributing to this pathology. Instead, chondrocytes are localized to the costosternal junction but there were no significant changes in the distribution of chondrocyte markers in this tissue, as evaluated by immunohistochemistry. These findings suggest that Clock or Npas2, and Bmal1, regulate ectopic calcification through a combination of systemic and local factors, and that the cells affected by Clock and Npas2, or Bmal1, disruption are a subset of the cells distributed in specific tissues that develop age-dependent arthropathy. The significance of these findings is that “circadian genes” play a role in the regulation of ectopic calcification in a non-oscillator capacity. Understanding this new mechanism by which ectopic calcification is controlled could lead to novel approaches for the treatment of some human calcification diseases.