MAPK1 and cleidocranial dysplasia 1: Similarly, mice lacking the MAP2Ks in the ERK pathway, Map2k1 (MAP2K1, MEK1) and Map2k2 (MAP2K2, MEK2), in osteoprogenitors (Sp7Cre) showed low bone mass and a cleidocranial dysplasia-like phenotype (CCD), similar to that seen in mice and humans with runt-related transcription factor 2 (RUNX2) haploinsufficiency (Ge et al., 2007; Ge et al., 2009; Kim et al., 2019).