Table 1:

Key Resources

Reagent or Resource	Source	Identifier
Antibodies
Mouse monoclonal anti-alpha actinin 2 (Clone EA-53)	Sigma	A7811
Mouse monoclonal anti-Titin	DHSB	9D10
Mouse anti-Myomesin	DHSB	MYOM
Mouse anti-MYH	DHSB	A4.591
Rabbit anti-MYH6	ProteinTech	22281-1-AP
Rabbit anti-MYH7	ProteinTech	22280-1-AP
Goat anti-mouse 488	Life Technologies	A11001
Goat anti-rabbit 488	Life Technologies	A11034
Goat anti-mouse 568	Life Technologies	A11004
Goat anti-rabbit 568	Life Technologies	A11036
Goat anti-mouse 647	Life Technologies	A32728
Goat anti-rabbit 647	Life Technologies	A32733
Biological Samples
Bovine Serum Albumin	RPI	A30075-100
Chemicals, peptides, and recombinant proteins
Phalloidin Alexa Fluor 488	Invitrogen	A12379
Phalloidin Alexa Fluor 568	Invitrogen	A12380
Phalloidin Alexa Fluor 647	Invitrogen	A22287
PBS, 10X, Ca2+/Mg2+ free	Life Technologies	70011-044
PBS, 10X, with Ca2+/Mg2+	Corning	46-013-CM
PFA, 16%	Electron Microscopy Sciences	15710
0.5% Trypsin	Life Technologies	15400-054
0.1% Gelatin	Sigma	ES-006-B
Dimethyl Sulfoxide	Sigma	276855
Vectashield with DAPI	Vector	H-1200
Blebbistatin	Sigma	B0560
Fibronectin	Corning	354008
Lipofectamine RNAiMAX	ThermoFisher	LMRNA015
TBS, 10X	Corning	46-012-CM
Tween 20	Sigma	P7949
Experimental Models: cell lines
iCell cardiomyocytes^2 kit	Fujifilm International	CMC-100-012-000.5
Oligonucleotides
SMART Pool siRNA against human MYH7 (13-16)	Horizon Discovery	A-011086
SMART Pool siRNA against human MYH7 (13, 14)	Horizon Discovery	A-011086-13, A-011086-14
siRNA against human MYH6 3’ UTR (13)	Horizon Discovery	A-012645-13-0005
siRNA against human MYH6 CDS (14)	Horizon Discovery	A-012645-14-0005
siRNA against human myomesin 3’UTR (13)	Horizon Discovery	A-006342-13-0005
siRNA against human myomesin CDS (16)	Horizon Discovery	A-006342-16-0005
Software and algorithms
Fiji	NIH
sarcApp	This study
yoU-Net	This study