Tendons and ligaments insert into bone through a transitional tissue termed the enthesis which is susceptible to injury and difficult to repair. Entheses contain a region of calcified fibrocartilage (CFC), however mineral-associated proteins in this tissue remain poorly characterized. Bone sialoprotein (BSP) is a phosphoprotein associated with mineralizing tissues. In these studies BSP was identified in the CFC of entheses by immunohistochemistry. Analysis of the entheses of Bsp-/- mice indicate abnormalities in the CFC. Compared to controls, the CFC of the quadriceps tendon enthesis is 28% and 41 % longer in 15 week and 14 month old Bsp-/- mice, respectively. MicroCT and Raman spectroscopic analysis of the CFC in Bsp-/- mice demonstrate that mineral content is similar between genotypes. Mechanical studies show that the Bsp-/- patellar tendon is larger in cross-sectional area yet mechanically weaker. These data suggest BSP is involved in the regulation and growth of the CFC.