RUNX2 and cleidocranial dysplasia 1: Currently, the Human Gene Mutation Database (HGMD Professional 2020.4 http://www.hgmd.cf.ac.uk/) has 202 registered RUNX2 variants responsible for CCD with various severities, namely, missense/nonsense (80), splicing (11), small/gross deletions (69), small insertions (28), gross insertions/duplications (8), indels (2), and complex rearrangements (4).