CTSK and obesity due to melanocortin 4 receptor deficiency: Previous studies from our own group and others have documented an elevation of cathepsin K expression/activity in association with adipocyte differentiation (Xiao et al., 2006; Funicello et al., 2007), high-fat diet-induced obesity (Yang et al., 2008), obesity-associated cardiac dysfunction (Hua et al., 2013b), cardiac hypertrophy, and heart failure (Cheng et al., 2006; Hua et al., 2013a).