SOX9 and Andersen-Tawil syndrome: Furthermore, induced pluripotent stem cells derived from ATS patients have decreased expression of key chondrogenic markers, including SOX9, which is a pro-chondrogenic transcription factor considered the master regulator of chondrocyte development, and RUNX2 and COLL10A1, which are SOX9-regulated targets and themselves critical downstream effectors in bone development (reviewed in Green et al., 2015) (Green et al., 2015).