MAPK1 and cleidocranial dysplasia 1: We generated mice lacking Mek1 and Mek2 in osteoprogenitors (Mek1OsxMek2−/−) by crossing mice with a conditional deletion of Mek1 in osteoprogenitors (Mek1Osx) and mice with a germline deletion of Mek2 (Mek2−/−), demonstrating that inactivation of ERK in osteoprogenitors results in severe bone loss in both axial and appendicular bones along with characteristic CCD phenotypes.