Image: Rainbow iPSCs three days after inducing the rainbow labeling. Clonally expanding iPSCs from the same parental origin are apparent as clusters of same-colored cells.

Danny El-Nachef, Darrian Bugg, Kevin M. Beussman, Sonette Steczina, Amy M. Martinson, Charles E. Murry, Nathan J. Sniadecki, Jennifer Davis

Circulation 2021; 143:1635–1638; DOI:


Preclinical studies have suggested that transplanted human pluripotent stem cell–derived cardiomyocyte (hPSC-CM) grafts expand because of proliferation.1 This knowledge came from cell cycle activity measurements that cannot discriminate between cytokinesis or DNA synthesis associated with hypertrophy. To refine our understanding of hPSC-CM cell therapy, we genetically engineered a cardiomyocyte-specific fluorescent barcoding system into an hPSC line. Because cellular progeny have the same color as parental hPSC-CMs, we could identify subsets of engrafted hPSC-CMs with greater clonal expansion.