Various human single- and double-headed β-cardiac myosin constructs. Expressed human β-cardiac myosin constructs (single-headed: S1; double-headed with varying heptad [HEP] tail length: 2HEP, 15HEP, and 25HEP). All constructs have a regulatory light chain (RLC), essential light chain (ELC), and a C-terminal EGFP, with the double-headed constructs also containing a GCN4 leucine zipper to ensure dimerization. Only the 15HEP and 25HEP can adopt the IHM conformation, which presumably stabilizes the SRX state.