MAPK9 and Osteopenia: Specifically, JNK is important for late-stage osteoblast differentiation, as inactivation of JNK by a specific small molecule inhibitor causes a lack of mineralization in MC3T3-E1 cells [56], while deletion of JNK1 and JNK2 in mice (Jnk1-2osx) results in severe osteopenia arising from bone loss [57].