Zaunbrecher RJ, Abel AN, Beussman K, et al. - Given that the leading known cause of familial dilated cardiomyopathy is mutations in the gene encoding the giant sarcomere protein titin, TTN, and that there is an uneven distribution of these mutations within TTN, researchers sought for a more clear understanding of this gene and the isoforms it encodes in cardiomyocyte sarcomere formation and function. The function of titin in human cardiomyocytes (CMs) was examined by employing CRISPR/Cas9 to produce homozygous truncations in the Z-disk (TTN-Z^-/-) and A-band (TTN-A^-/-) regions of the TTN gene in human induced pluripotent stem cells. Following differentiation, it was noted that despite the more upstream mutation, TTN-Z^-/--CMs had sarcomeres and visibly contracted, while TTN-A^-/--CMs did not. Sarcomere formation is assumed to be due to the expression of Cronos, a recently discovered isoform of titin, which inducts downstream of the truncation in TTN-Z^-/--CMs. Findings suggest that developing human fetal cardiac tissue profoundly expresses Cronos titin, and when knocked out in hiPSC-CMs, these cells show decreased contractile force and myofibrillar disarray, despite the presence of full-length titin and that Cronos is able to support partial sarcomere formation in the absence of full-length titin. Further, it was observed that for proper sarcomere function in hiPSC-CMs, Cronos titin is crucial.