RUNX2 and cleidocranial dysplasia 1: 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].