Similarly, mice expressing a dominant-negative mutant of the Map2k Mek1 in mature osteoblasts exhibit hypomineralization of clavicle and calvaria [5], similar to cleidocranial dysplasia (CCD) seen in human patients with loss-of-function mutations in RUNX2 or mice with Runx2 haploinsufficiency [15,16]. This evidence concerns the gene MAP2K1 and cleidocranial dysplasia 1.