Schematic diagram of a myosin II molecule. SM1 and SM2 heterodimers are drawn for illustration purposes only. Controversy exists about whether in living cells they exist as homodimers (Kelley et al., 1992; Rovner et al., 2002) or homodimers and heterodimers by random chance (Tsao and Eddinger, 1993). S1 and S2, HMM subfragments. Adapted from Chen et al. (2018) and Kumar and Mansson (2017).