TG and hypothyroidism: This developmental expression of 2B MyHC is likely a response to the neonatal surge of thyroid hormone, since hypothyroidism in the adult uniquely suppresses 2B MyHC expression in fibres of fast primary ontotype, replacing it with β-slow MyHC (Zhong et al. 2010), as the inhibitory action of thyroid hormone on β-slow MyHC gene is removed.