The finding that Atp2a2 heterozygosity does not exacerbate the effects of hypothyroidism in mice may result from the switch in myosin heavy chain (MHC) isoforms from α-MHC to the slower β-MHC, which occurs in hypothyroidism, and is known to be energetically favorable [52]. The gene discussed is ATP2A2; the disease is hypothyroidism.