TGFB1 and chronic obstructive pulmonary disease: Consistent with the functional analysis, TGF-β signaling module showed significant correlations with increased skeletal muscle nitrosative stress in COPD patients (|R| = 0.49, P = 0.031), as well as with abnormally low levels of blood valine amino acids (|R| = 0.41, P = 0.047).