After 2– 3 weeks of culture, most cells expressed vimentin (SR: 161/177, 92.4 ± 3.8%, n = 3; AF: 234/249, 89.4 ± 7.7%, n = 4) and human fibroblast surface protein (hFSP; SR: 136/143, 95.5 ± 2.4%, n = 3; AF: 174/185, 94.2 ± 1.8%, n = 4), confirming their fibroblast phenotype (Fig. 1B). Here, GZMA is linked to atrial fibrillation.