Rac1 is expressed in both mouse and human cartilage. (A) Robust expression of Rac1 was detected at the surface and middle zones of mouse cartilage but chondrocytes in calcified zone displayed reduced expression. (B) Rac1 is distributed ubiquitously in human articular cartilage. Arrows in both panels indicate representative Rac1-positive chondrocytes. Scale bars = 50 μm.