SOX9 and atrial fibrillation: We also investigated the knockdown effect of MKX in human AF (HAF) primary cultured cells and found that the expression of tendon/ligament-related genes, such as those encoding SCX, TNMD, COL14A1, TNC, BGN and TNXB, was significantly downregulated, whereas expression of the cartilage-related genes SOX9 and ACAN was upregulated (Fig. 3b).