In order to examine the role of Hsp27 in the molecular mechanisms underlying tenderness, we generated an HspB1-null mouse (Kammoun et al., 2013). The mutant mouse was viable, fertile and showed neither apparent morphological noranatomical alterations. The macroscopic or microscopic muscle phenotype was not altered. However, there was evidence for a muscle-type specific alteration of the molecular phenotype in relation to 1) apoptosis, Hsp status and anti-oxidant status in an oxidative muscle and 2) Hsp status and calcium homeostasis in a glycolytic muscle. Lastly, a preliminary transmission electron microscopy experiment revealed ultrastructural abnormalities in the myofibrillar structure of mutant mice. These data suggest that Hsp27 could directly impact the organisation of the muscle cytoskeleton and contribute to the determinism of tenderness at the molecular and ultrastructural levels.