During culture, the diameter of the pellets decreased over time in pellets cultured without or with a low dose of BMP-4 (0.04 ​nM) in both NP and AF cells (Supplementary Fig. 1), whereas pellets cultured with 0.4 ​nM BMP-4 maintained the same diameter throughout the culture period. This evidence concerns the gene BMP4 and atrial fibrillation.