COL9A1 and Hip dysplasia: Among these, known associated genes were TBX4 and NOTCH2. We further found very damaging mutations in three collagen genes: COL12A1, COL2A1, and COL9A1. Mutations in COL2A1 are known to cause spondyloepiphyseal dysplasia and COL2A1 has previously been related to osteoarthritis secondary to DDH (Granchi et al. 2002).