Using medium containing TGFβ3 [37] [43] that was previously used to differentiate primary AC and NP cells [22, 24], we were able to differentiate between two subsets of immortal AF clones based on their ability to process COL1A1: AF-S subclones rapidly produced mature Collagen forms that propelled 3D cellular sheet formation and contraction, while a second group of AF clones (AF-nS) did this to a much lower extent. This evidence concerns the gene COL1A1 and atrial fibrillation.