GYS1 and type 2 diabetes mellitus: T2DM animals present several conditions that favor glycogenesis such as (1) elevated levels of insulin, which stimulate glycogen synthesis through inactivation of glycogen synthase kinase 3 (GSK-3) that is known to maintain GYS1 in an activated form; (2) hyperglycaemia, which increases the blood-to-testicles glucose availability, exerting an allosteric activation on GYS1 via glucose-6-phosphate (accelerating glycogen synthesis) [55, 56]; (3) increased testicular levels of UDP-glucose, an active intermediate in the synthesis of glycogen [25].