RepSox hydrochloride is a potent and selective inhibitor of transforming growth factor-β receptor I/activin-like kinase 5 (TGF-β-RI/ALK5). RepSox hydrochloride inhibits ALK5 autophosphorylation with an IC50 value of 4 nM. RepSox hydrochloride can be used in the study of obesity and related metabolic diseases such as type 2 diabetes.