However, a study of the comparative biology of mammalian telomeres indicated that telomere length correlates inversely, rather than directly, with lifespan, and concluded that the contribution of telomere length to lifespan remains controversial.[2] Also, telomere shortening does not occur with age in some postmitotic tissues, such as in the rat brain.[3] In humans, skeletal muscle telomere lengths remain stable from ages 23–74.[4] In baboon skeletal muscle, that consists of fully differentiated post-mitotic cells, less than 3% of myonuclei contain damaged telomeres and this percentage does not increase with age.[5] Thus telomere shortening does not appear to be a major factor in the aging of the differentiated cells of brain or skeletal muscle.