ACAN and atrial fibrillation: Transcription factor SOX9 conditionally deleted in aggrecan expressing cells. Progressive degeneration of the end plates and extracellular matrix remodeling in both the NP and AF, consistent with human disc degeneration Tsingas et al. (2020). SOX9 null discs showed the presence of non-vacuolated NP cells Tsingas et al. (2020), while conditional SOX9 deletion results in NC death in developing notochord Barrionuevo et al. (2006).