MSTN and obesity due to melanocortin 4 receptor deficiency: Myostatin is a member of the transforming growth factor-β family, and although it is abundant in muscles, it is also found in small amounts in adipose tissue and heart muscle.[8] Some studies revealed an association between increased myostatin levels and increased age, decreased muscle mass, and muscle strength.[9,10] In addition, high myostatin levels have been shown to be associated with obesity and insulin resistance (IR).[11,12]